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.

Stability and Lattice Dynamics of Ruddlesden–Popper Tetragonal Sr2TiO4

“…The structural relaxation was performed in the primitive body-centered tetragonal unit cell, until the maximum residual forces on each atom were less than 6x10 -6 Ha/Bohr and the pressure was less than 2x10 -8 Ha/Bohr 3 . From our DFT calculations with the LDA exchange-correlation functional, we find lattice parameters a = 3.8217 Å and c = 12.3145 Å, which are about 2 % lower than our experimental results and prior results in the literature 49,51 . In contrast, our c/a ratios are almost the same (c/a = 3.2223 in DFT against c/a = 3.241 in our experiments and 3.241 49 , 3.247 51 in prior experiments).…”

“…From our DFT calculations with the LDA exchange-correlation functional, we find lattice parameters a = 3.8217 Å and c = 12.3145 Å, which are about 2 % lower than our experimental results and prior results in the literature 49,51 . In contrast, our c/a ratios are almost the same (c/a = 3.2223 in DFT against c/a = 3.241 in our experiments and 3.241 49 , 3.247 51 in prior experiments). Our results agree well with prior LDA calculations performed by Fennie and Rabe.…”

“…41 Beyond infrared and Raman techniques, preliminary inelastic neutron scattering experiments have also been reported. 48 Finally, the high-temperature thermal expansion (a vibrational-related property) of Sr2TiO4 was recently studied by neutron powder diffraction (NPD) and Synchrotron X-ray diffraction (XRD) experiments 49 as well as DFT calculations. 43,47 In this work, we report an extensive study of the lattice dynamics in Sr2TiO4 as a function of pressure and temperature.…”

A systematic evaluation of the role of lanthanide elements in functional complex oxides; implications for energy conversion devices

“…An earlier synchrotron XRD measurement by Kawamura et al has shown a similar difference in the LaSrAlO 4 Ruddlesden-Popper material, where the La-O electronic interaction is stronger than the Sr-O electronic interaction. 62 Quantitatively, the calculated La-O electron density overlap in LaAlO 3 was weaker than the one observed from the experiments. Such differences between the computational and experimental results have been reported in the literature, e.g.…”

The Flexible Unit Structure Engine (FUSE) for probe structure-based composition prediction

“…This Extended Module Materials Assembly (EMMA) method 10 was employed to aid in the determination of the structure of a large layered perovskite, Y 2. 24 21 , where the modular approach worked well to impose chemically sensible co-ordination chemistry on all of the atoms.…”