title: Structural origin of the anisotropic and isotropic thermal expansion of K2NiF4-Type LaSrAlO4 and Sr2TiO4.
abstract: K2NiF4-type LaSrAlO4 and Sr2TiO4 exhibit anisotropic and isotropic thermal expansion, respectively; however, their structural origin is unknown. To address this unresolved issue, the crystal structure and thermal expansion of LaSrAlO4 and Sr2TiO4 have been investigated through high-temperature neutron and synchrotron X-ray powder diffraction experiments and ab initio electronic calculations. The thermal expansion coefficient (TEC) along the c-axis (αc) being higher than that along the a-axis (αa) of LaSrAlO4 [αc = 1.882(4)αa] is mainly ascribed to the TEC of the interatomic distance between Al and apical oxygen O2 α(Al-O2) being higher than that between Al and equatorial oxygen O1 α(Al-O1) [α(Al-O2) = 2.41(18)α(Al-O1)]. The higher α(Al-O2) is attributed to the Al-O2 bond being longer and weaker than the Al-O1 bond. Thus, the minimum electron density and bond valence of the Al-O2 bond are lower than those of the Al-O1 bond. For Sr2TiO4, the Ti-O2 interatomic distance, d(Ti-O2), is equal to that of Ti-O1, d(Ti-O1) [d(Ti-O2) = 1.0194(15)d(Ti-O1)], relative to LaSrAlO4 [d(Al-O2) = 1.0932(9)d(Al-O1)]. Therefore, the bond valence and minimum electron density of the Ti-O2 bond are nearly equal to those of the Ti-O1 bond, leading to isotropic thermal expansion of Sr2TiO4 than LaSrAlO4. These results indicate that the anisotropic thermal expansion of K2NiF4-type oxides, A2BO4, is strongly influenced by the anisotropy of B-O chemical bonds. The present study suggests that due to the higher ratio of interatomic distance d(B-O2)/d(B-O1) of A2(2.5+)B(3+)O4 compared with A2(2+)B(4+)O4, A2(2.5+)B(3+)O4 compounds have higher α(B-O2), and A2(2+)B(4+)O4 materials exhibit smaller α(B-O2), leading to the anisotropic thermal expansion of A2(2.5+)B(3+)O4 and isotropic thermal expansion of A2(2+)B(4+)O4. The "true" thermal expansion without the chemical expansion of A2BO4 is higher than that of ABO3 with a similar composition.
authors.forename: Keishi
authors.initials: K
authors.lastname: Kawamura
authors.forename: Masatomo
authors.initials: M
authors.lastname: Yashima
authors.forename: Kotaro
authors.initials: K
authors.lastname: Fujii
authors.forename: Kazuki
authors.initials: K
authors.lastname: Omoto
authors.forename: Keisuke
authors.initials: K
authors.lastname: Hibino
authors.forename: Shuntaro
authors.initials: S
authors.lastname: Yamada
authors.affiliation: §Bragg Institute, Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia.
authors.forename: James R
authors.initials: JR
authors.lastname: Hester
authors.affiliation: §Bragg Institute, Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia.
authors.forename: Maxim
authors.initials: M
authors.lastname: Avdeev
authors.affiliation: #Neutron Science Laboratory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan.
authors.forename: Ping
authors.initials: P
authors.lastname: Miao
authors.affiliation: #Neutron Science Laboratory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan.
authors.forename: Shuki
authors.initials: S
authors.lastname: Torii
authors.affiliation: #Neutron Science Laboratory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan.|⊥The Graduate University for Advanced Studies (SOKENDAI), SOKENDAI-KEK Office, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan.
authors.forename: Takashi
authors.initials: T
authors.lastname: Kamiyama