Equation of state of CaMnO3: a combined experimental and computational study

Wojciech Paszkowicz 0 1 2 3 Scott M. Woodley 0 1 2 3 Pawe Piszora 0 1 2 3 Bohdan Bojanowski 0 1 2 3 Jarosaw Pietosa 0 1 2 3 Yngve Cerenius 0 1 2 3 Stefan Carlson 0 1 2 3 Christine Martin 0 1 2 3 0 P. Piszora Dept of Materials Chemistry, Faculty of Chemistry, Adam Mickiewicz University , Grunwaldzka 6, 60-780 Poznan, Poland 1 S.M. Woodley ( ) Kathleen Lonsdale Materials Chemistry, Dept of Chemistry, University College London , 20 Gordon Str., London WC1H 0AJ, UK 2 C. Martin Laboratoire CRISMAT-ENSICAEN (UMR CNRS 6805) , CNRS, 6, bld Marchal Juin, 14050 Caen Cedex 04, France 3 B. Bojanowski Institute of Physics, West Pomeranian University of Technology , al . Piastw 48, 70-311 Szczecin, Poland Elastic properties of CaMnO3 are of primary importance in the science and technology of CaMnO3-based perovskites. From X-ray diffraction experiments performed at pressures up to 100 kbar using a diamond-anvil cell to hydrostatically compress our sample, a bulk modulus, K0, of 1734(96) kbar was obtained after fitting parameters to the third-order Birch-Murnaghan equation of state. Mean field, semiclassical simulations predict, for the first time, the thirdorder equation-of-state parameters and show how the bulk modulus increases with pressure (the zero pressure value being 2062.1 kbar) and decreases with the extent of nonstoichiometry caused by the formation of oxygen vacancies. These trends are amplified for the shear modulus. A more accurate model that allows for the explicit reduction of Mn ions, or localization of excess electrons, yields qualitatively similar results. The experimental and calculated axial ratios show the same trends in their variation with rising pressure. - ABO3 perovskite oxides, (A = divalent alkaline-earth or trivalent rare-earth ion, B = transition metal ion) have been extensively studied in recent years. For materials of this class, physical properties are related to the occurrence of phenomena such as a JahnTeller distortion, a variation of the valence states of the transition metal ions, charge ordering, ionic conductivity, ferromagnetism, ferroelectricity, piezoelectricity, and the colossal magnetoresistance. Other phenomena such as a metal-to-insulator transition, magnetic-field-dependent structural transition, anomalous thermal conductivity temperature dependence or the isotopic effect on the Curie temperature, TC, have also been reported for some materials of this class. Properties of perovskites are strongly pressure dependent. Pressure influences the transport properties; it can suppress the Jahn Teller distortion and modify the conductivity type [1]. It changes the crucial structural quantities such as BO bond lengths and BOB bond angles. The BOB bond angles influence the strength of double exchange interaction responsible for ferromagnetic coupling in Mn-containing perovskite oxides. In general, these bond angles have been found to increase under applied pressure, leading to an enhancement of the double exchange interactions and a rise of TC (in the range up to 1520 kbar) [2]; at larger pressures, the behavior is more complicated [35]. CaMnO3 (orthorhombic, space group Pnma), a member of the ABO3 family, is a parent compound for numerous multicomponent Mn-based perovskite oxides exhibiting colossal magnetoresistance, such as La1 x Cax MnO3 and Sr1 x Cax MnO3. Mixed-valence manganites have the potential for applications based on their chemical and physical properties. Recently, doped CaMnO3 has been considered as a material useful in thermoelectric power generation, in particular for waste heat recovery [68]. Elastic properties of CaMnO3 under high pressure have not been rigorously studied up to now. The reported values of experimental and calculated equation-of-state (EOS) parameters, K0, K , and V0, exhibit large scatter. For example, the experimental value for K0 is reported to be 528 kbar (as determined using ultrasonic measurements) [9], 1544(33) kbar (obtained by XRD at pseudohydrostatic conditions) [10], 1710 kbar (conditions close to hydrostatic in the whole pressure range studied) [11] and 2240(250) kbar (with hydrostatic conditions applied in a part of the fitting range) [12]. The importance of using hydrostatic compression and the suitability of pressure transmitting medium (PTM) formed from an alcohol-mixture for this purpose results from various recent experimental studies [1315] (the hydrostaticity limit for this PTM is about 100 kbar). It is worth noting that for a related material, SrMnO3, three experiments differing by measurement conditions led to large discrepancies in bulkmodulus value (see [16]; the authors briefly discuss some possible reasons and remedies). Two EOS parameters (a0 and K0) have been reported in each of two theoretical approaches, DFT [17] and LMTO [18]. In three more recent simulations [9, 19, 20], only the bulk modulus has been calculated. In the cited papers, K0 is predicted to be 675 kbar (Heterogeneous Metal Mixture model, HMM) [9], 1514.9 kbar and 2545.8 kbar (Modified Rigid Ion Model, MRIM) [20], 2114 kbar (Linear Muffin Tin Orbitals, LMTO) [18], 2150 kbar (Density Functional Theory, DFT) [17] and 3253.8 kbar (Born model) [19]. The discrepancies in both experimental and predicted values of bulk modulus are so large that the necessity of new investigation is evident; naturally the new experimental study must be performed at hydrostatic conditions within a possibly broad pressure range including the ambient pressure. In this work, elastic properties of a stoichiometric CaMnO3 sample are studied at high pressures. A pV EOS is determined by fitting the diffraction data collected as a function of pressure. Furthermore, semi-classical simulations are employed in order to investigate the dependence of bulk modulus on pressure. Next, the discrepancies between the experimentally observed behavior and the theoretical predictions are discussed on the basis, in particular, of (demonstrated by simulations) trends taking place in the incorporation of oxygen vacancies. 2 Experimental Our stoichiometric CaMnO3 sample was synthesized in air in the form of a small bar at 1300 C, starting from stoichiometric ratios of pure CaO and Mn2O3. The Ca:Mn ratio was controlled using X-ray energy-dispersive spectrometry (EDS) with accuracy of 0.02. As verified by measurements of the electric transport and magnetic properties, ideal oxygen stoichiometry was found and, moreover, the EDS of the cationic composition confirmed the cation homogeneity in the sample. The refined lattice parameters determined for the same sample (space group Pnma) are a = 5.28159(4) , b = 7.45730(4) , c = 5.26748(4) , which gives V = 207.467(4) [21]. The high-pressure diffraction experiments were performed using a membrane-driven diamond anvil cell (DXRGM, EasyLab Technologies Ltd.), equipped with diamond anvils of 0.3 mm culet diameter. The sample was loaded into a hole (0.15 mm diameter, 0.1 mm depth) in the stainlesssteel gasket placed in-be (...truncated)