[1] SCOTT J F. Applications of modern ferroelectrics[J]. Science, 315, 954(2007).

[2] YANG S Y, SEIDEL J, BYRNES S J et al. Above-bandgap voltages from ferroelectric photovoltaic devices[J]. Nature Nanotechnology, 5, 143(2010).

[3] CHEONG S W, MOSTOVOY M. Multiferroics: a magnetic twist for ferroelectricity[J]. Nature Materials, 6:, 13(2007).

[4] WANG K F, LIU J M, REN Z F. Multiferroicity: the coupling between magnetic and polarization orders[J]. Advances in Physics, 58, 321(2009).

[5] HILL N A. Why are there so few magnetic ferroelectrics?[J]. The Journal of Physical Chemistry B, 104, 6694(2000).

[6] WANG J, NEATON J B, ZHENG H et al. Epitaxial BiFeO₃ multiferroic thin film heterostructures[J]. Science, 299, 1719(2003).

[7] DONG S, LIU J M, CHEONG S W et al. Multiferroic materials and magnetoelectric physics: symmetry, entanglement, excitation, and topology[J]. Advances in Physics, 64, 519(2015).

[8] KIMURA T, GOTO T, SHINTANI H et al. Magnetic control of ferroelectric polarization[J]. Nature, 426, 55(2003).

[9] GOODENOUGH J B. Theory of the role of covalence in the perovskite-type manganites [La,M(II)]MnO₃[J]. Physical Review, 100, 564(1955).

[10] MILLIS A J. Lattice effects in magnetoresistive manganese perovskites[J]. Nature, 392, 147(1998).

[11] LEVANYUK A P, SANNIKOV D G. Anomalies in dielectric properties in phase transitions[J]. Zhurnal Eksperimental'noi i Teoreticheskoi Fiziki, 55:, 256(1968).

[12] LEVANYUK A P, SANNIKOV D G. Improper ferroelectrics[J]. Uspekhi Fizicheskih Nauk, 112, 561(1974).

[13] DVOŘÁK V. Improper ferroelectrics[J]. Ferroelectrics, 7, 1(1974).

[14] BOUSQUET E, DAWBER M, STUCKI N et al. Improper ferroelectricity in perovskite oxide artificial superlattices[J]. Nature, 452, 732(2008).

[15] BENEDEK N A, FENNIE C J. Hybrid improper ferroelectricity: a mechanism for controllable polarization-magnetization coupling[J]. Physical Review Letters, 106, 107204(2011).

[16] ZHANG Y J, WANG J, GHOSEZ P. Unraveling the suppression of oxygen octahedra rotations in A₃B₂O₇ Ruddlesden-Popper compounds: engineering multiferroicity and beyond[J]. Physical Review Letters, 125, 157601(2020).

[17] WANG F, GAO H, DE GRAAF C et al. Switchable Rashba anisotropy in layered hybrid organic-inorganic perovskite by hybrid improper ferroelectricity[J]. npj Computational Materials, 6:, 183(2020).

[18] VARIGNON J, BRISTOWE N C, GHOSEZ P. Electric field control of Jahn-Teller distortions in bulk perovskites[J]. Physical Review Letters, 116, 057602(2016).

[19] TIAN H, KUANG X Y, MAO A J et al. Novel type of ferroelectricity in brownmillerite structures: a first-principles study[J]. Physical Review Materials, 2, 084402(2018).

[20] STROPPA A, BARONE P, JAIN P et al. Hybrid improper ferroelectricity in a multiferroic and magnetoelectric metal- organic framework[J]. Advanced Materials, 25, 2284(2013).

[21] BRUCE A D, COWLEY R A, BURNS G. Structural phase transitions[J]. Physics Today, 34, 58(1981).

[22] BRUCE A D. Universal phenomena near structural phase transitions[J]. Ferroelectrics, 35, 43(1981).

[23] YOUNG J, STROPPA A, PICOZZI S et al. Anharmonic lattice interactions in improper ferroelectrics for multiferroic design[J]. Journal of Physics: Condensed Matter, 27, 283202(2015).

[24] XU B, WANG D W, ZHAO H J et al. Hybrid improper ferroelectricity in multiferroic superlattices: finite-temperature properties and electric-field-driven switching of polarization and magnetization[J]. Advanced Functional Materials, 25, 3626(2015).

[26] MULDER A T, BENEDEK N A, RONDINELLI J M et al. Turning ABO₃ antiferroelectrics into ferroelectrics: design rules for practical rotation-driven ferroelectricity in double perovskites and A₃B₂O₇ Ruddlesden-Popper compounds[J]. Advanced Functional Materials, 23, 4810(2013).

[27] ZHAO H, LIU X, CHEN X M et al. Effects of chemical and hydrostatic pressures on structural, magnetic, and electronic properties of R₂NiMnO₆ (R=rare-earth ion) double perovskites[J]. Physical Review B, 90:, 195147(2014).

[28] SCHAAK R E, MALLOUK T E. Perovskites by design: a toolbox of solid-state reactions[J]. Chemistry of Materials, 14, 1455(2002).

[29] BENEDEK N A, RONDINELLI J M, DJANI H et al. Understanding ferroelectricity in layered perovskites: new ideas and insights from theory and experiments[J]. Dalton Transactions, 44, 10543(2015).

[30] CHEN X M, XIAO Y, LIU X Q et al. SrLnAlO₄ (Ln=Nd and Sm) microwave dielectric ceramics[J]. Journal of Electroceramics, 10, 111(2003).

[31] ISHIDA K, MUKUDA H, KITAOKA Y et al. Spin-triplet superconductivity in Sr₂RuO₄ identified by ¹⁷O Knight shift[J]. Nature, 396:, 658(1998).

[32] LEE D, LEE H. Controlling oxygen mobility in Ruddlesden- Popper oxides[J]. Materials, 10, 368(2017).

[33] SENN M S, BOMBARDI A, MURRAY C A et al. Negative thermal expansion in hybrid improper ferroelectric Ruddlesden- Popper perovskites by symmetry trapping[J]. Physical Review Letters, 114, 035701(2015).

[34] TU D, XU C N, KAMIMURA S et al. Ferroelectric Sr₃Sn₂O₇: Nd³⁺: a new multipiezo material with ultrasensitive and sustainable near-infrared piezoluminescence[J]. Advanced Materials, 32, 1908083(2020).

[35] RUDDLESDEN S N, POPPER P. New compounds of the K₂NIF₄ type[J]. Acta Crystallographica, 10, 538(1957).

[36] RUDDLESDEN S N, POPPER P. The compound Sr₃Ti₂O₇ and its structure[J]. Acta Crystallographica, 11, 54(1958).

[37] RONDINELLI J M, FENNIE C J. Octahedral rotation-induced ferroelectricity in cation ordered perovskites[J]. Advanced Materials, 24, 1961(2012).

[38] TARANCÓN A, BURRIEL M, SANTISO J et al. Advances in layered oxide cathodes for intermediate temperature solid oxide fuel cells[J]. Journal of Materials Chemistry, 20, 3799(2010).

[39] CHRONEOS A, YILDIZ B, TARANCÓN A et al. Oxygen diffusion in solid oxide fuel cell cathode and electrolyte materials: mechanistic insights from atomistic simulations[J]. Energy & Environmental Science, 4, 2774(2011).

[40] ZHANG B H, LIU X Q, CHEN X M. Review of experimental progress of hybrid improper ferroelectricity in layered perovskite oxides[J]. Journal of Physics D: Applied Physics, 55, 113001(2022).

[41] LI C F, ZHENG S H, WANG H W et al. Structural transitions in hybrid improper ferroelectric Ca₃Ti₂O₇ tuned by site-selective isovalent substitutions: a first-principles study[J]. Physical Review B, 97, 184105(2018).

[42] GAO B, HUANG F T, WANG Y Z et al. Interrelation between domain structures and polarization switching in hybrid improper ferroelectric Ca₃(Mn, Ti)₂O₇[J]. Applied Physics Letters, 110, 222906(2017).

[43] LIU X Q, WU J W, SHI X X et al. Hybrid improper ferroelectricity in Ruddlesden-Popper Ca₃(Ti, Mn)₂O₇ ceramics[J]. Applied Physics Letters, 106, 202903(2015).

[44] ZHANG B H, HU Z Z, CHEN B H et al. Improved hybrid improper ferroelectricity in B-site substituted Ca₃Ti₂O₇ ceramics with a Ruddlesden-Popper structure[J]. Journal of Applied Physics, 128, 054102(2020).

[45] CHEN B H, SUN T L, WEI L Y et al. Enhanced hybrid improper ferroelectricity in Fe/Nb cosubstituted Ca₃Mn₂O₇ ceramics[J]. Journal of the American Ceramic Society, 104, 4000(2021).

[46] ELCOMBE M M, KISI E H, HAWKINS K D et al. Structure determinations for Ca₃Ti₂O₇, Ca₄Ti₃O₁₀, Ca_3.6Sr_0.4Ti₃O₁₀ and a refinement of Sr₃Ti₂O₇[J]. Acta Crystallographica Section B Structural Science, 47, 305(1991).

[47] GUIBLIN N, GREBILLE D, LELIGNY H et al. Ca₃Mn₂O₇[J]. Acta Crystallographica Section C, 58(2002).

[48] LOBANOV M V, GREENBLATT M, CASPI E A N et al. Crystal and magnetic structure of the Ca₃Mn₂O₇Ruddlesden- Popper phase: neutron and synchrotron X-ray diffraction study[J]. Journal of Physics: Condensed Matter, 16, 5339(2004).

[49] OH Y S, LUO X, HUANG F T et al. Experimental demonstration of hybrid improper ferroelectricity and the presence of abundant charged walls in (Ca, Sr)₃Ti₂O₇ crystals[J]. Nature Materials, 14, 407(2015).

[50] MEIER D, SEIDEL J, CANO A et al. Anisotropic conductance at improper ferroelectric domain walls[J]. Nature Materials, 11, 284(2012).

[51] GAO P, BRITSON J, JOKISAARI J R et al. Atomic-scale mechanisms of ferroelastic domain-wall-mediated ferroelectric switching[J]. Nature Communications, 4:, 2791(2013).

[52] WU W D, HORIBE Y, LEE N et al. Conduction of topologically protected charged ferroelectric domain walls[J]. Physical Review Letters, 108, 077203(2012).

[53] GUREEV M Y, TAGANTSEV A K, SETTER N. Head-to-head and tail-to-tail 180° domain walls in an isolated ferroelectric[J]. Physical Review B, 83, 184104(2011).

[54] HUANG X R, HU X B, JIANG S S et al. Theoretical model of 180° domain-wall structures and their transformation in ferroelectric perovskites[J]. Physical Review B, 55, 5534(1997).

[55] KURUSHIMA K, YOSHIMOTO W, ISHII Y et al. Direct observation of charged domain walls in hybrid improper ferroelectric (Ca, Sr)₃Ti₂O₇[J]. Japanese Journal of Applied Physics, 56(2017).

[56] SMITH K A, NOWADNICK E A, FAN S et al. Infrared nano-spectroscopy of ferroelastic domain walls in hybrid improper ferroelectric Ca₃Ti₂O₇[J]. Nature Communications, 10:, 5235(2019).

[57] LEE M H, CHANG C P, HUANG F T et al. Hidden antipolar order parameter and entangled Néel-type charged domain walls in hybrid improper ferroelectrics[J]. Physical Review Letters, 119, 157601(2017).

[58] NAKAJIMA H, SHIGEMATSU K, HORIBE Y et al. Charged domain walls and crystallographic microstructures in hybrid improper ferroelectric Ca_3-xSr_xTi₂O₇[J]. Materials Transactions, 60, 2103(2019).

[59] MUNRO J M, AKAMATSU H, PADMANABHAN H et al. Discovering minimum energy pathways via distortion symmetry groups[J]. Physical Review B, 98, 085107(2018).

[60] NOWADNICK E A, FENNIE C J. Domains and ferroelectric switching pathways in Ca₃Ti₂O₇ from first principles[J]. Physical Review B, 94, 104105(2016).

[61] HUANG F T, GAO B, KIM J W et al. Topological defects at octahedral tilting plethora in bi-layered perovskites[J]. npj Quantum Materials, 1:, 16017(2016).

[62] KRATOCHVILOVA M, HUANG F T, DIAZ M F et al. Mapping the structural transitions controlled by the trilinear coupling in Ca_3-xSr_xTi₂O₇[J]. Journal of Applied Physics, 125, 244102(2019).

[63] POMIRO F, ABLITT C, BRISTOWE N C et al. From first- to second-order phase transitions in hybrid improper ferroelectrics through entropy stabilization[J]. Physical Review B, 102:, 014101(2020).

[64] HU Z Z, LU J J, CHEN B H et al. First-order phase transition and unexpected rigid rotation mode in hybrid improper ferroelectric (La, Al) co-substituted Ca₃Ti₂O₇ ceramics[J]. Journal of Materiomics, 5, 618(2019).

[65] NAGAI T, MOCHIZUKI Y, SHIRAKUNI H et al. Phase transition from weak ferroelectricity to incipient ferroelectricity in Li₂Sr(Nb_1-xTa_x)₂O₇[J]. Chemistry of Materials, 32, 744(2020).

[66] HALASYAMANI P S, POEPPELMEIER K R. Noncentrosymmetric oxides[J]. Chemistry of Materials, 10, 2753(1998).

[67] MALLICK S, FORTES A D, ZHANG W G et al. Switching between proper and hybrid-improper polar structures via cation substitution in A₂La(TaTi)O₇ (A=Li, Na)[J]. Chemistry of Materials, 33, 2666(2021).

[68] CAMMARATA A, RONDINELLI J M. Ferroelectricity from coupled cooperative Jahn-Teller distortions and octahedral rotations in ordered Ruddlesden-Popper manganates[J]. Physical Review B, 92:, 014102(2015).

[69] BENEDEK N A. Origin of ferroelectricity in a family of polar oxides: the Dion-Jacobson phases[J]. Inorganic Chemistry, 53, 3769(2014).

[70] WU X X, WANG S Y, WONG-NG W et al. Novel optical properties and induced magnetic moments in Ru-doped hybrid improper ferroelectric Ca₃Ti₂O₇[J]. Journal of Advanced Ceramics, 10, 120(2021).

[71] HUANG C, WONG-NG W, LIU W F et al. Major improvement of ferroelectric and optical properties in Na-doped Ruddlesden- Popper layered hybrid improper ferroelectric compound, Ca₃Ti₂O₇[J]. Journal of Alloys and Compounds, 770:, 582(2019).

[72] JIANG Y, WANG S Y, LEI Y L et al. Negative piezoelectric behaviors in hybrid improper ferroelectric Ca_3-xNa_xTi₂O₇ (x=0, 0.01) ceramics[J]. Journal of the American Ceramic Society, 103, 4429(2020).

[73] LI G J, LIU X Q, LU J J et al. Crystal structural evolution and hybrid improper ferroelectricity in Ruddlesden-Popper Ca_3-xSr_xTi₂O₇ ceramics[J]. Journal of Applied Physics, 123, 014101(2018).

[74] LI S T, BIROL T. Suppressing the ferroelectric switching barrier in hybrid improper ferroelectrics[J]. npj Computational Materials, 6:, 168(2020).

[75] JACOB K T, GUPTA S. Phase diagram of the system Ca-Ti-O at 1200 K[J]. Bulletin of Materials Science, 32, 611(2009).

[76] JACOB K T, ABRAHAM K P. Thermodynamic properties of calcium titanates: CaTiO₃, Ca₄Ti₃O₁₀, and Ca₃Ti₂O₇[J]. The Journal of Chemical Thermodynamics, 41, 816(2009).

[77] GONG W P, WU L L, NAVROTSKY A. Combined experimental and computational investigation of thermodynamics and phase equilibria in the CaO-TiO₂ system[J]. Journal of the American Ceramic Society, 101, 1361(2018).

[78] PFAFF G. Synthesis of calcium titanate powders by the Sol-Gel process[J]. Chemistry of Materials, 6, 58(1994).

[79] ZHOU C, CAI W, ZHANG Q W et al. Enhancement in hybrid improper ferroelectricity of Ca₃Ti₂O₇ ceramics by a two-stage sintering[J]. Materials Chemistry and Physics, 258:, 124001(2021).

[80] WU H D, CAI W, ZHOU C et al. Remarkable enhancement in hybrid improper ferroelectricity of Ca₃Ti₂O₇ ceramics by a simple Sol-Gel process[J]. Materials Letters, 278:, 128447(2020).

[81] LI X, YANG L, LI C F et al. Ultra-low coercive field of improper ferroelectric Ca₃Ti₂O₇ epitaxial thin films[J], 110, 042901(2017).

[82] LU X Z, RONDINELLI J M. Epitaxial-strain-induced polar-to- nonpolar transitions in layered oxides[J]. Nature Materials, 15, 951(2016).

[83] SHI Y, WANG S Y, MA S et al. Nanoscale imaging of ferroelectric domain and resistance switching in hybrid improper ferroelectric Ca₃Ti₂O₇ thin films[J]. Physics Letters A, 384, 126609(2020).

[84] CHEN B H, SUN T L, LIU X Q et al. Enhanced hybrid improper ferroelectricity in Sr_3-xBa_xSn₂O₇ ceramics with a Ruddlesden- Popper (R-P) structure[J]. Applied Physics Letters, 116, 042903(2020).

[85] LU J J, LIU X Q, MA X et al. Crystal structures, dielectric properties, and phase transition in hybrid improper ferroelectric Sr₃Sn₂O₇-based ceramics[J]. Journal of Applied Physics, 125, 044101(2019).

[86] HU Z Z, LU J J, CHEN B H et al. Improved ferroelectric properties in hybrid improper ferroelectric Sr_3-xBa_xZr₂O₇[J]. Journal of Alloys and Compounds, 866:, 158705(2021).

[87] HUANG L F, LU X Z, RONDINELLI J M. Tunable negative thermal expansion in layered perovskites from quasi-two-dimensional vibrations[J]. Physical Review Letters, 117, 115901(2016).

[88] ABLITT C, MCCAY H, CRADDOCK S et al. Tolerance factor control of uniaxial negative thermal expansion in a layered perovskite[J]. Chemistry of Materials, 32, 605(2020).

[89] SENN M S, MURRAY C A, LUO X et al. Symmetry switching of negative thermal expansion by chemical control[J]. Journal of the American Chemical Society, 138, 5479(2016).

[90] GLAMAZDA A, WULFERDING D, LEMMENS P et al. Soft tilt and rotational modes in the hybrid improper ferroelectric Ca₃Mn₂O₇[J]. Physical Review B, 97, 094104(2018).

[91] YE F, WANG J C, SHENG J M et al. Soft antiphase tilt of oxygen octahedra in the hybrid improper multiferroic Ca₃Mn_1.9Ti_0.1O₇[J]. Physical Review B, 97, 041112(2018).

[92] LIU S, ZHANG H, GHOSE S et al. Nature of the structural symmetries associated with hybrid improper ferroelectricity in Ca₃X₂O₇ (X=Mn and Ti)[J]. Physical Review B, 99, 224105(2019).

[93] CHERIAN J G, BIROL T, HARMS N C et al. Optical spectroscopy and band gap analysis of hybrid improper ferroelectric Ca₃Ti₂O₇[J]. Applied Physics Letters, 108, 262901(2016).

[94] TONG B Y, WANG S Y, WONG-NG W et al. Polarization switching dynamics and switchable diode effect in hybrid improper ferroelectric Ca₃Ti₂O₇ ceramics[J]. Journal of the American Ceramic Society, 102, 1875(2019).

[95] OMARI L H, LEMZIOUKA H, MOUBAH R et al. Structural and optical properties of Fe-doped Ruddlesden-Popper Ca₃Ti_2-xFe_xO_7-δ nanoparticles[J]. Materials Chemistry and Physics, 246:, 122810(2020).

[96] WANG F Q, CAI W, FU C L et al. The electronic structure and optical properties of Ca₃(Mn_1-xTi_x)₂O₇ from first-principle calculations[J]. Journal of Advanced Dielectrics, 9, 1950007(2019).

[97] ZHANG X N, LIU W F, HAN Y L et al. Novel optical and magnetic properties of Li-doped quasi-2D manganate Ca₃Mn₂O₇ particles[J]. Journal of Materials Chemistry C, 5, 7011(2017).

[98] ZHANG J T, SHEN X F, WANG Y C et al. Design of two- dimensional multiferroics with direct polarization-magnetization coupling[J]. Physical Review Letters, 125, 017601(2020).

[99] HARRIS A B. Symmetry analysis for the Ruddlesden-Popper systems Ca₃Mn₂O₇ and Ca₃Ti₂O₇[J]. Physical Review B, 84, 064116(2011).

[100] ZHU W K, PI L, HUANG Y J et al. Electrically induced decrease of magnetization in Ca₃Mn₂O₇[J]. Applied Physics Letters, 101, 192407(2012).

[101] LU J B, MA C. Oxygen octahedral coupling and structural reconstruction at the intergrowth interface in bilayered Ca₃Mn₂O₇[J]. Acta Materialia, 129:, 26(2017).

[102] JUNG W H. Weak ferromagnetism of n=2 Ruddlesden-Popper Ca₃Mn₂O₇ system[J]. Journal of Materials Science Letters, 19:, 2037(2000).

[103] HUANG F T, XUE F, GAO B et al. Domain topology and domain switching kinetics in a hybrid improper ferroelectric[J]. Nature Communications, 7:, 11602(2016).

[104] BARANOVSKIY A, AMOUYAL Y. Structural stability of calcium-manganate based CaO(CaMnO₃)_m (m=1, 2, 3, ∞) compounds for thermoelectric applications[J]. Journal of Alloys and Compounds, 687:, 562(2016).

[105] LIU M F, ZHANG Y, LIN L F et al. Direct observation of ferroelectricity in Ca₃Mn₂O₇ and its prominent light absorption[J]. Applied Physics Letters, 113, 022902(2018).

[106] LI S Y, WANG S Y, LU Y G et al. Exchange bias effect in hybrid improper ferroelectricity Ca_2.94Na_0.06Mn₂O₇[J]. AIP Advances, 8, 015009(2018).

[107] BARROZO P, SMÅBRÅTEN D R, TANG Y L et al. Defect- enhanced polarization switching in the improper ferroelectric LuFeO₃[J]. Advanced Materials, 32, e2000508(2020).

[108] MORIYA T. New mechanism of anisotropic superexchange interaction[J]. Physical Review Letters, 4, 228(1960).

[109] COCHRAN W. Crystal stability and the theory of ferroelectricity[J]. Physical Review Letters, 3, 412(1959).

[110] PEREZ-MATO J M, AROYO M, GARCÍA A et al. Competing structural instabilities in the ferroelectric Aurivillius compound SrBi₂Ta₂O₉[J]. Physical Review B, 70, 214111(2004).

[111] ZEMP Y, TRASSIN M, GRADAUSKAITE E et al. Magnetoelectric coupling in the multiferroic hybrid-improper ferroelectric Ca₃Mn_1.9Ti_0.1O₇[J]. Physical Review B, 109, 184417(2024).

[112] XU X H, WANG Y Z, HUANG F T et al. Highly tunable ferroelectricity in hybrid improper ferroelectric Sr₃Sn₂O₇[J]. Advanced Functional Materials, 30, 2003623(2020).

[113] CHEN Q S, ZHANG B H, CHEN B H et al. Distortion modes and ferroelectric properties in hybrid improper ferroelectric Sr₃(Sn, Zr)₂O₇ ceramics[J]. Journal of Applied Physics, 131, 184102(2022).

[114] KAMIMURA S, YAMADA H, XU C N. Strong reddish-orange light emission from stress-activated Sr_n+1Sn_nO_3n+1:Sm³⁺ (n=  1, 2, ∞) with perovskite-related structures[J]. Applied Physics Letters, 101, 091113(2012).

[115] GREEN M A, PRASSIDES K, DAY P et al. Structure of the n=2 and n=∞ member of the Ruddlesden-Popper series, Sr_n+1Sn_nO_3n+1[J]. International Journal of Inorganic Materials, 2, 35(2000).

[116] BENEDEK N A, MULDER A T, FENNIE C J. Polar octahedral rotations: a path to new multifunctional materials[J]. Journal of Solid State Chemistry, 195:, 11(2012).

[117] WANG Y Z, HUANG F T, LUO X et al. The first room-temperature ferroelectric Sn insulator and its polarization switching kinetics[J]. Advanced Materials, 29, 1601288(2017).

[118] YOSHIDA S, AKAMATSU H, TSUJI R et al. Hybrid improper ferroelectricity in (Sr, Ca)₃Sn₂O₇ and beyond: universal relationship between ferroelectric transition temperature and tolerance factor in n=2 Ruddlesden-Popper phases[J]. Journal of the American Chemical Society, 140, 15690(2018).

[119] YOSHIDA S, FUJITA K, AKAMATSU H et al. Ferroelectric Sr₃Zr₂O₇: competition between hybrid improper ferroelectric and antiferroelectric mechanisms[J]. Advanced Functional Materials, 28, 1801856(2018).

[120] LIU X Q, LU J J, CHEN B H et al. Hybrid improper ferroelectricity and possible ferroelectric switching paths in Sr₃Hf₂O₇[J]. Journal of Applied Physics, 125, 114105(2019).

[121] GUO Z, ZHANG Z D, LIU X Q et al. Hybrid improper ferroelectricity in La₂Sr(Sc_1-xFe_x)₂O₇ ceramics with double-layered Ruddlesden-Popper structures[J]. Applied Physics Letters, 125, 042902(2024).

[122] ZHANG R H, ABBETT B M, READ G et al. La₂SrCr₂O₇: controlling the tilting distortions of n=2 Ruddlesden-Popper phases through A-site cation order[J]. Inorganic Chemistry, 55, 8951(2016).

[123] YI W, KAWASAKI T, ZHANG Y et al. La₂SrSc₂O₇: A-site cation disorder induces ferroelectricity in Ruddlesden-Popper layered perovskite oxide[J]. Journal of the American Chemical Society, 146, 4570(2024).

[124] FLOROS N, MICHEL C, HERVIEU M et al. New n = 2 members of the Li₂Sr_n+1M_nO_3n+1 family, closely related to the Ruddlesden-Popper phases: structure and non-stoichiometry[J]. Journal of Materials Chemistry, 9, 3101(1999).

[125] GALVEN C, FOURQUET J L, SUARD E et al. Mechanism of a reversible CO₂ capture monitored by the layered perovskite Li₂SrTa₂O₇[J]. Dalton Transactions, 39, 4191(2010).

[126] GALVEN C, MOUNIER D, PAGNIER T et al. Thermal structural characterization of the acentric layered perovskite LiHSrTa₂O₇: X-ray and neutron diffraction, SHG and Raman experiments[J]. Dalton Transactions, 43, 14841(2014).

[127] SINGH S K, MURTHY V R K. Effect of crystal structure on microwave dielectric properties of Li₂SrTa_2(1-x)Nb_2xO₇ compounds[J]. Materials Research Bulletin, 70:, 514(2015).

[128] SINGH S K, MURTHY V R K. Microwave dielectric properties of Li₂SrTa_2(1-x)Nb_2xO₇ ceramics investigated by Raman spectroscopy[J]. Ceramics International, 42, 7284(2016).

[129] UPPULURI R, AKAMATSU H, GUPTA A S et al. Competing polar and antipolar structures in the Ruddlesden-Popper layered perovskite Li₂SrNb₂O₇[J]. Chemistry of Materials, 31, 4418(2019).

[130] NAGAI T, SHIRAKUNI H, NAKANO A et al. Weak ferroelectricity in n=2 pseudo Ruddlesden-Popper-type niobate Li₂SrNb₂O₇[J]. Chemistry of Materials, 31, 6257(2019).

[131] PAGNIER T, ROSMAN N, GALVEN C et al. Phase transition in the Ruddlesden-Popper layered perovskite Li₂SrTa₂O₇[J]. Journal of Solid State Chemistry, 182, 317(2009).

[132] MOCHIZUKI Y, NAGAI T, SHIRAKUNI H et al. Coexisting mechanisms for the ferroelectric phase transition in Li₂SrNb₂O₇[J]. Chemistry of Materials, 33, 1257(2021).

[133] LIANG Z H, TANG K B, SHAO Q et al. Synthesis, crystal structure, and photocatalytic activity of a new two-layer Ruddlesden-Popper phase, Li₂CaTa₂O₇[J]. Journal of Solid State Chemistry, 181, 964(2008).

[134] GALVEN C, MOUNIER D, BOUCHEVREAU B et al. Phase transitions in the Ruddlesden-Popper phase Li₂CaTa₂O₇: X-ray and neutron powder thermodiffraction, TEM, Raman, and SHG experiments[J]. Inorganic Chemistry, 55, 2309(2016).

[135] ZHANG B H, HU Z Z, CHEN B H et al. Room-temperature ferroelectricity in A-site ordered Ruddlesden-Popper Li₂CaTa₂O₇ ceramics[J]. Journal of Materiomics, 6, 593(2020).

[136] PITCHER M J, MANDAL P, DYER M S et al. Tilt engineering of spontaneous polarization and magnetization above 300 K in a bulk layered perovskite[J]. Science, 347, 420(2015).

[137] BATTLE P D, MILLBURN J E, ROSSEINSKY M J et al. Neutron diffraction study of the structural and electronic properties of Sr₂HoMn₂O₇ and Sr₂YMn₂O₇[J]. Chemistry of Materials, 9, 3136(1997).

[138] ZHANG R H, SENN M S, HAYWARD M A. Directed lifting of inversion symmetry in Ruddlesden-Popper oxide-fluorides: toward ferroelectric and multiferroic behavior[J]. Chemistry of Materials, 28, 8399(2016).

[139] GUPTA A S, AKAMATSU H, STRAYER M E et al. Improper inversion symmetry breaking and piezoelectricity through oxygen octahedral rotations in layered perovskite family, LiRTiO₄ (R= rare earths)[J]. Advanced Electronic Materials, 2, 1500196(2016).

[140] GUPTA A S, AKAMATSU H, BROWN F G et al. Competing structural instabilities in the Ruddlesden-Popper derivatives HRTiO₄ (R=rare earths): oxygen octahedral rotations inducing noncentrosymmetricity and layer sliding retaining centrosymmetricity[J]. Chemistry of Materials, 29, 656(2017).

[141] AKAMATSU H, FUJITA K, KUGE T et al. A-site cation size effect on oxygen octahedral rotations in acentric Ruddlesden- Popper alkali rare-earth titanates[J]. Physical Review Materials, 3, 065001(2019).

[142] AKAMATSU H, FUJITA K, KUGE T et al. Inversion symmetry breaking by oxygen octahedral rotations in the Ruddlesden-Popper NaRTiO₄ family[J]. Physical Review Letters, 112, 187602(2014).

[143] RODGERS J A, BATTLE P D, DUPRÉ N et al. Cation and spin ordering in the n=1 Ruddlesden-Popper phase La₂Sr₂LiRuO₈[J]. Chemistry of Materials, 16, 4257(2004).

[144] NISHIMOTO S, MATSUDA M, HARJO S et al. Structure determination of n=1 Ruddlesden-Popper compound HLaTiO₄ by powder neutron diffraction[J]. Journal of the European Ceramic Society, 26, 725(2006).

[145] NISHIMOTO S, MATSUDA M, HARJO S et al. Structural change in a series of protonated layered perovskite compounds, HLnTiO₄ (Ln=La, Nd and Y)[J]. Journal of Solid State Chemistry, 179, 1892(2006).

[146] BYEON S H, YOON J J, LEE S O. A new family of protonated oxides HLnTiO₄ (Ln=La, Nd, Sm, and Gd)[J]. Journal of Solid State Chemistry, 127, 119(1996).

[147] SILYUKOV O I, ABDULAEVA L D, BUROVIKHINA A A et al. Phase transformations during HLnTiO₄ (Ln=La, Nd) thermolysis and photocatalytic activity of obtained compounds[J]. Journal of Solid State Chemistry, 226:, 101(2015).

[148] BALACHANDRAN P V, PUGGIONI D, RONDINELLI J M. Crystal-chemistry guidelines for noncentrosymmetric A₂BO₄ Ruddlesden-Popper oxides[J]. Inorganic Chemistry, 53, 336(2014).

[149] ZHANG B H, XU D M, CHEN B H et al. Hybrid improper ferroelectricity in A-site cation ordered Li₂La₂Ti₃O₁₀ ceramic with triple-layer Ruddlesden-Popper structure[J]. Applied Physics Letters, 118, 052903(2021).

[150] ZHANG B H, XU D M, GUO R Z et al. Hybrid improper ferroelectricity and phase transition behavior of Li₂Nd₂Ti₃O₁₀ ceramics with A-site ordered triple-layer Ruddlesden-Popper structure[J]. Journal of Materiomics, 10, 145(2024).