New germanates RCrGeO5 (R=Nd–Er, Y): Synthesis, structure, and properties
Graphical abstract
Alternating CrCr separations in the chain of edge-sharing CrO6 octahedra and the constraining effect of the GeO5 square pyramids in the structures of RCrGeO5.
Introduction
Complex manganese oxides RMn2O5 (R=Y, Bi, or rare-earth cations) have recently attracted considerable attention due to an unusual magnetoelectric effect [1], [2], [3], [4], [5], [6], [7], [8], [9]. The key feature of these compounds deals with magnetic frustration resulting in a number of phase transitions to ordered (commensurately or incommensurately) spin states, while some of these states give rise to polar structural distortions and ferroelectricity. Thus, the physics of RMn2O5 is controlled by the magnitudes of competing magnetic interactions, whereas the interactions depend on structural parameters and, in particular, on the size of the R cation. Large R cations (La, Bi) provide relatively simple commensurate magnetic structures [10], [11]. Smaller cations (Y, SmLu) give rise to more complicated spin states and magnetic field-controlled ferroelectricity [12].
The temperature of the ferroelectric transitions in the RMn2O5 compounds is rather low (30–40 K), and the realization of the magnetoelectric effect in these systems at higher temperatures remains a challenging problem. One may think that the substitution of manganese by other transition metal cations will modify the spin system hence leading to higher transition temperatures. Indeed, the temperatures of long-range magnetic ordering of RFeMnO5 (R=Y, Ho, Er) are enhanced as compared with that of RMn2O5. However, the RFeMnO5 compounds are weakly frustrated, since different relevant orbitals of the penta-coordinated cation (Fe+3 in RFeMnO5 and Mn+3 in RMn2O5) give rise to magnetic interactions of different sign. Moreover, a partial Fe/Mn disorder is present, and the RFeMnO5 oxides do not show ferroelectricity [13]. Another RMn2O5-type compound, YCrMnO5, strongly suffers from Cr/Mn disorder and is also unsuitable for the purpose formulated above [14]. Nevertheless, the search for new RMn2O5-type compounds is of high importance, if one succeeds to combine two cations giving rise to magnetic frustration, and to provide complete ordering of these cations within the RMn2O5 structure.
To realize the RMn2O5-type structure, it is necessary to choose two different cations M′ and M″ having stable octahedral and square pyramidal coordination, respectively (see Fig. 1). Basically, the choice of the cations is not limited by transition metals only. For example, RAlGeO5 compounds include Al as M′ and Ge as M″ [15], [16], while YGa1−xMn1+xO5 contains Ga atoms that predominantly occupy the square-pyramidal position [17]. In our study, we try another combination of the cations and use Cr+3 as M′ due to its high stability in air. As for M″, we choose Ge+4. The substitution of Mn atoms in RMn2O5 by magnetic Cr+3 and non-magnetic Ge+4 results in a series of novel compounds RCrGeO5. We study the prepared compounds with X-ray powder diffraction (XPD), electron microscopy, magnetic susceptibility and specific heat measurements focusing on the details of the crystal structure. Next, we use the structural data to discuss the factors influencing the stability of the RMn2O5 structure and suggest new ideas for combining appropriate transition metal cations within this structure type.
Section snippets
Experimental
Bulk powder samples of RCrGeO5 (R=NdEr, Y) were obtained by solid-state reaction of stoichiometric mixtures of R2O3, Cr2O3, and GeO2 in air for 2 weeks at 1250° C with several intermediate grindings. Initial oxides were intimately grinded in an agate mortar under acetone, pressed into pellets and placed into alumina crucibles. After 1 week of annealing only RCrGeO5 and RCrO3 (about 5 wt%) were found in the reaction mixture. The formation of the RCrO3 impurity implies the lack of GeO2, likely due
Crystal structure of the RCrGeO5 compounds
X-ray patterns for all the RCrGeO5 compounds are very similar. The patterns were indexed in orthorhombic symmetry with lattice parameters listed in Table 1. The analysis of the systematic extinctions allowed us to suggest the Pbam space group similar to the RMn2O5 phases (R=LaLu, Y) [20]. This conclusion was confirmed by ED study (see below). We failed to obtain single crystals of RCrGeO5 due to phase decomposition above 1300 °C. Therefore, the crystal structures were refined using XPD data. The
Discussion
RCrGeO5 germanates present new examples of RMn2O5-type compounds. Chromium atoms occupy octahedral positions (like Mn4+ or Al3+), while germanium ones are placed in the square pyramid (like Mn3+). Note that the RMn2O5 compounds were synthesized for all rare-earth cations as well as for yttrium and bismuth [10], [20], whereas RCrGeO5 germanates could be prepared for a limited number of R only (NdEr). The different stability of RMn2O5 and RCrGeO5 oxides may be explained by different nature of the
Acknowledgment
The authors are grateful to RFBR (Grant 07-03-00890) for financial support. R.Sh. is grateful to NIMS for granting his stay. A.Ts. acknowledges the hospitality of MPI CPfS (Dresden) and the financial support. Part of this work has been performed within the framework of the IAP V-1 of the Belgian government.
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