Poisson ratio and bulk lattice constant of (Sr0.25La0.75)CrO3 from strained epitaxial thin films

Abstract

About 30 nm thick (001)-oriented (Sr0.25La0.75)CrO3 (SLCO) epitaxial thin films were grown by solid-source oxide molecular beam epitaxy on four different single-crystalline cubic or pseudo-cubic (001)-oriented oxide substrates: LaAlO3, (LaAlO3)0.3(Sr2AlTaO6)0.7, SrTiO3 and DyScO3, which result in lattice mismatch ranging from-2% to +1.7%. All the films are of high-quality, flat and strained by the substrates. By assessing the evolution of the out-of-plane lattice parameter as a function of the in-plane lattice parameter of the samples, we determine both the Poisson ratio (ν = 0.32) and the bulk lattice constant (ab = 3.876 Å) of SLCO. The Poisson ratio significantly differs from LaCrO3 (ν = 0.23) and the (SrxLa1-x)CrO3 solid solution appears to obey structural Vegard's law. Since SLCO is the only one p-type transparent conductive oxide of perovskite structure and has promising thermoelectric properties, integrating SLCO in heterostructures and devices is therefore of paramount importance, which confers on our results their strong interest. Besides, the method used here can be straightforwardly applied to other complex oxides.

Giant Tuning of Electronic and Thermoelectric Properties by Epitaxial Strain in p-Type Sr-Doped LaCrO3 Transparent Thin Films

“…The LSCO transmittance appears very similar to other emergent perovskite TCO, such as n-type SrVO3 for instance (<65% with 45 nm thick films) 4 , and somehow lower than standard TCO such as n-type Indium-Tin Oxide (ITO) but which is limited by indium scarcity and not of perovskite structure 5,7 . -c) 27 . Figure 2a shows the 2/ scans recorded around the (002) reflections of the LSCO films and the substrates.…”

“…2b), ii) the film composition has been checked by XPS to be at the targeted stoichiometry, although the instrumental uncertainty is larger than 5 at.% (Supporting Information, Fig. S6) 11,27 , iii) proper flux measurements and composition calibrations have been performed with our best accuracy (less than 3 at.%, Supporting Information, Fig. S1) 27 , iv) the transport properties of LSCO film on SrTiO3(001) substrate are in perfect agreement with literature and the strain-dependent trend (Supporting Information, Figs.…”

“…The out-of-plane strain (εzz) of the LSCO films ranges from +1.68% (in-plane compressive) to -1.86% (in-plane tensile). Elastic strain is rather large in LSCO because of its relatively large Poisson ratio (ν = 0.32) 27 . Figure 2c shows that the unit-cell volume of LSCO films (VLSCO, blue triangles in Figure 2c) increases from 56.8 Å 3 (compressive) to 59.2 Å 3 (tensile) 27 .…”

“…The fluxes were calibrated using a Bayard-Alpert Gauge and checked with a quartz microbalance in UHV (Fig. S1) 27,56 . The substrates were annealed under the growth conditions for 15 minutes to remove surface impurities before starting the growth at 750 °C and P(O2) = 1×10 -7 Torr, and at a growth rate of 0.15 nm per minute.…”

“…While the electrical conductivity (σ) and Seebeck coefficient (S) of La1-xSrxCrO3 can be tuned by the partial aliovalent cationic substitution ratio x on the A-site of the perovskite structure, which forms shallow impurity energy level and associated mobile hole carriers 23,54 , the corresponding thermoelectric power factor (PF = S 2 σ) being maximum for x around 0.25 keeping rather good optical transparency 23,24 . At room temperature, La0.75Sr0.25CrO3 (LSCO) is a transparent thermoelectric semiconductor with a orthorhombic bulk structure (a = 5.503 Å, b = 5.467 Å, c = 7.76 Å, and <Cr-O-Cr> bond angles around 162°, both rather temperature-independent), pseudo-cubic bulk lattice constant of 3.876 Å, and within 25% Cr 4+ 3d t2g 2 electronic configuration 24,27,55 . No study has been reported yet concerning the strain impact on p-type Sr-doped LaCrO3 properties, despite its technological importance as p-type TEO and TCO and the strain-engineering potential.…”