Equation of state of CaMnO_{3}: a combined experimental and computational study
Abstract
Elastic properties of CaMnO_{3} are of primary importance in the science and technology of CaMnO_{3}-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, K_{0}, 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 third-order 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.
1 Introduction
ABO_{3} 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 Jahn–Teller 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, T_{C}, 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 B–O bond lengths and B–O–B bond angles. The B–O–B 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 T_{C} (in the range up to 15–20 kbar) [2]; at larger pressures, the behavior is more complicated [3, 4, 5].
CaMnO_{3} (orthorhombic, space group Pnma), a member of the ABO_{3} family, is a parent compound for numerous multicomponent Mn-based perovskite oxides exhibiting colossal magnetoresistance, such as La_{1−x}Ca_{x}MnO_{3} and Sr_{1−x}Ca_{x}MnO_{3}. Mixed-valence manganites have the potential for applications based on their chemical and physical properties. Recently, doped CaMnO_{3} has been considered as a material useful in thermoelectric power generation, in particular for waste heat recovery [6, 7, 8]. Elastic properties of CaMnO_{3} under high pressure have not been rigorously studied up to now. The reported values of experimental and calculated equation-of-state (EOS) parameters, K_{0}, K′, and V_{0}, exhibit large scatter. For example, the experimental value for K_{0} 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 [13, 14, 15] (the hydrostaticity limit for this PTM is about 100 kbar). It is worth noting that for a related material, SrMnO_{3}, three experiments differing by measurement conditions led to large discrepancies in bulk-modulus value (see [16]; the authors briefly discuss some possible reasons and remedies).
Two EOS parameters (a_{0} and K_{0}) 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, K_{0} 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 CaMnO_{3} sample are studied at high pressures. A p–V 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 CaMnO_{3} sample was synthesized in air in the form of a small bar at 1300 °C, starting from stoichiometric ratios of pure CaO and Mn_{2}O_{3}. 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) Å, b=7.45730(4) Å, c=5.26748(4) Å, which gives V=207.467(4) [21].
The high-pressure diffraction experiments were performed using a membrane-driven diamond anvil cell (DXR-GM, 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 stainless-steel gasket placed in-between the diamonds. The data were collected at MAX-Lab (Lund, Sweden) I711 beamline [22] with wavelength of 0.91985 Å, using a MAR165 detector. The FIT2D program [23] was used for conversion of the 2D data to 1D. A 16:3:1 methanol-ethanol-water mixture [24] was used as the PTM. The small amount of sample in respect to the PTM volume was intentional, in order to minimize stresses connected with interaction between grains. The pressure was determined using laser-excited ruby luminescence. Lattice parameters were calculated from positions of nonoverlapping peaks using the UNITCELL program [25]. For CaMnO_{3}, the chosen approach for determination of lattice parameters from positions of nonoverlapping lines is appropriate for getting reliable lattice parameter values; to obtain a good accuracy, a synchrotron beam is used to ascertain good statistics, which is necessary because of the weak intensity of the peaks. The experimental Birch–Murnaghan (BM) EOS [26, 27] was fitted to 11 experimental V(p) points below 100 kbar representing the hydrostatic range for the PTM applied, using the EOSFIT program [28, 29].
3 Theoretical approach
In our simulations, calculation of the lattice energy and physical properties for relaxed structures of various stoichiometries under a number of fixed pressures were performed using the General Utility Lattice Program (GULP) [30]; more details of our model including the interatomic potential parameters can be found in [21]. In fact, two mean-field approaches were employed to model the orthorhombic CaMnO_{3−δ} compounds. Reducing the occupancy of the oxygen sites to (3−δ)/3 gives rise to an unphysical positive charge (for δ>0) on the unit cell. This charge is counter-balanced by either introducing a uniform neutralizing charge background (−8δ|e|) across the unit cell or explicitly modeling a reduction of manganese by decreasing the mean charge on the manganese sublattice from 4 to (4−2δ)|e|. We will refer to these approaches as the “fixed” and “variable” charge models, respectively.
4 Results
4.1 Equation of state: experimental results
In the analysis of the experimental results, it is convenient to compare the (dimensionless) axial ratios. The present experiment shows a virtually constant value of \(b/(a\sqrt{2}\,)\) in the considered pressure range, whereas for c/a a slight tendency to increase from 0.9973 to 0.9987 is visible in Fig. 2. The slopes of all three experimental lines are apparently very small and almost identical. The (small) discrepancy in the absolute value of these lines, for both axial-ratios, can be due either to the difference in sample composition or to differences in lattice-parameter calculation methods. The marginal variation of axial ratios with pressure shows that the compression of CaMnO_{3} is almost isotropic in the studied pressure range.
V_{0} (Å^{3}) | K_{0} (kbar) | K′ | Experiment details | Assumed EOS type | Reference | Year | ||
---|---|---|---|---|---|---|---|---|
Experimental method | PTM | Pressure range (kbar) | ||||||
207.5^{a} | 1710 | 5.9 | ADXRD | nitrogen | 15.6–386 (hr) | BM3 | [11] | 2003 |
208.39 | 528 | – | ultrasonic | – | – | – | [9] | 2008 |
208.0(1) | 2240(25) | 6.0(1.8) | ADXRD | AWM | 0–365 (hlr) | M fit | [12] | 2010 |
207.29(7) | 1544(33) | 4 | EDXRD | pseudo-hydrostatic setting | 0–48.4 (ph) | BM2 | [10] | 2011 |
207.55(11) | 1734(96) | 4.8(2.2) | ADXRD | AWM | 0–100 (hr) | BM3 | this work | 2012 |
Interestingly, our experimental value for K_{0} is (very) close to that reported for isostructural solid solutions with as high as 50 or 75 % occupation of the Ca site by a rare earth: K_{0}=1860(50) or 2100 kbar for La_{0.5}Ca_{0.5}MnO_{3} [11, 31], K_{0}=1720(20) kbar for Nd_{0.5}Ca_{0.5}MnO_{3} [32], and K_{0}=1780 kbar for La_{0.75}Ca_{0.25}MnO_{3} [33]. However, for full La occupation, the bulk modulus value is strongly reduced to 1080(20) [1] or 1040 kbar [11], whereas for PrMnO_{3} and SmMnO_{3} larger values of 1639 and 1695 bar can be derived from [34] (the latter may be affected by nonhydrostatic compression conditions applied). More detailed analysis of such trends would require a dedicated study with identical experimental conditions applied.
4.2 Equation of state from simulations: results for stoichiometric case
The pressure variation of unit-cell volume is accompanied by insignificant changes in the axial ratios: c/a increases by 0.0003, while \(b/(a\sqrt{2}\,)\) decreases by 0.0007, over 100 kbar. The following observations were made for the axial ratios, which are dimensionless and, therefore, are less dependent, than absolute values, on the exact fit of the potential parameters of our model; simulated data for \(b/(a\sqrt{2}\,)\) are in perfect agreement with the experimental ones, whereas the for c/a there is a small (less than 0.8 %) discrepancy, cf. Fig. 2. The slopes of the lines are only marginally different.
From our simulations of CaMnO_{3}, the lattice parameters (after minimizing the lattice enthalpy under ambient conditions) relaxed to 5.297 Å, 7.486 Å and 5.242 Å (giving V_{0}=207.86 Å^{3}). The remaining parameters of the equation of state have the values of 2062.1 kbar for the bulk modulus, and 3.3 and 3.1 for its first derivative, \(K_{0}'\) calculated numerically using a step size of 1 and 0.2 kbar, respectively. The obtained value of the shear modulus is 997.1 kbar, which is three times as large as the experimental value of 349 kbar reported in [9] and twice as large as compared to the room-temperature value (439 kbar) found for Pr_{0.48}Ca_{0.52}MnO_{3} [35]. Note that the moduli are calculated from the elastic constants using Hill’s definition [30].
EOS parameters for CaMnO_{3} as determined from present and earlier simulations
V_{0} (Å^{3}) | K_{0} (kbar) | K′ | Simulation approach | Pressure range | Assumed EOS type | Reference | Year |
---|---|---|---|---|---|---|---|
210.94^{a} | 2150 | – | DFT, primitive cubic cell assumed | – | – | [17] | 1997 |
216.56^{a} | 2114 | – | LMTO | – | – | [18] | 1998 |
– | 3253.8 | – | Born model | – | – | [19] | 2006 |
– | 675 | – | HMM | – | – | [9] | 2008 |
– | 1514.9 (LT) | – | MRIM | – | – | [20] | 2009 |
– | 2545.8 (HT) | – | MRIM | – | – | [20] | 2009 |
207.86 | 2062.1 | 3.3 ($) | Born model | 0–100 | BM3 | this work | 2012 |
4.3 Equation of state: theoretical results for non-stoichiometric case
We performed calculations for the nonstoichiometric compound, CaMnO_{3−δ}, using the two different models described above. In both models, we assumed that there is no significant effect due to anion/vacancy ordering on the oxygen sites and that the effect of vacancies can be spread over all oxygen sites by averaging (which is described within the mean-field approach [30]).
In our simulations of the nonstoichiometric CaMnO_{3−δ}, we expect an underestimation of the orthorhombicity of the structure, i.e., greater differences between lattice parameters as the tetragonal cell distorts to orthorhombic upon minimization of lattice energy, as the inclusion of vacancies would also create Jahn–Teller Mn^{3+} ions, which we have not explicitly included in our model. However, we can easily model a spread of an average charge on the manganese sites (the variable charge model), rather than use a neutralizing charge background, i.e., we now consider the effect of localization of the additional electrons on the Mn sublattice. From our simulations of CaMnO_{2.7}, we obtain K_{0}=1445, S_{0}=758, K_{40}=1596, S_{40}=803, K_{100}=1798, and S_{100}=860 kbar. Thus, the softening of both K and S with increasing δ is much quicker—compare Figs. 5a and 5b (note that simulations were performed for δ=0.015n, where n is an integer, 0,1,2…20). For δ=0.03 (close to the highest observed experimental δ value), the charge of the Mn shells is set to 3.94|e|, as a consequence of which, the tilt of the octahedra increases (smaller Mn–O–Mn bond angles, which are now 156.52° and 157.61°), and K_{0} has already softened to 2004 kbar and, therefore, agrees better with our observed value. We remind the reader that the bulk modulus obtained for our sample was not used in the fit of our interatomic potential parameters. To obtain a value of 1734(96) kbar, without refining potential parameters, we need δ to be as high as 0.14. Of course, although the statistical model may be somewhat “naïve,” we can still ascertain the trend that oxygen vacancies will markedly reduce the value of K_{0}, while a decrease of the charge on the Mn sites leads to an increase of the tilt of the MnO_{6} octahedra. As expected, the calculations indicate that for nonstoichiometric (i.e., less densely packed) material, the K_{0} value is lower than for the stoichiometric compound.
5 Summary
In summary, high-pressure diffraction experiments for stoichiometric CaMnO_{3} were undertaken in order to determine, for the first time, its equation of state (EOS) at hydrostatic conditions from a clean experimental volume/pressure dependence. In addition, semiclassical simulations were also performed in order to determined the simulated third-order third-order EOS. This investigation was completed in order to shed light on the discrepancies existing among the earlier reported experimental and theoretical bulk-modulus values. The experimental bulk modulus of CaMnO_{3}, derived by fitting the Birch–Murnaghan EOS to the V_{exp}(p) data, is 1734(96) kbar. The simulated value of 2062.1 is only slightly lower than those from previous DFT and LMTO calculations; the discrepancy between theory and experiment is only about 15 %. Our experiment and simulations show how the calculated bulk modulus increases with pressure. Moreover, from simulations, we predict how the bulk modulus decreases with the extent of nonstoichiometry resulting from the presence of oxygen vacancies—bulk modulus decreases with lower atomic density whether caused by internal (change in stoichiometry) or external pressure. Allowing for the explicit reduction of manganese ions, simulations produce qualitatively similar results, but with higher gradients, a trend that is amplified for the shear modulus.
Notes
Acknowledgements
Professor C. Meneghini (Universita Roma Tre) is acknowledged for providing the unpublished data. The research leading to these results has received funding from both the European Community’s Seventh Framework Programme (FP7/2007-2013), under grant agreement no. 226716, and EPSRC (EP/F067496).
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