High-Temperature Magnetism in CaTcO₃ and SrTcO₃

Authors

Maxim Avdeev, Melody Carter and Gordon Thorogood (ANSTO), Tony Cheetham (Cambridge), Brendan Kennedy and Jimmy Ting (Sydney U.), Anna Llobet (Los Alamos), Frederic Poineau (University of Nevada Las Vegas), Efrain Rodriguez (NIST), Ram Seshadri (University of California Santa Barbara), David Singh (Oak Ridge), and Kia Wallwork (Australian Synchrotron)

Introduction

The materials of a general formula ABX3 with crystal structure of the perovskite type (Fig. 1, left) are technologically important due to the wide range of useful physical properties they possess, ranging from ferroelectricity and piezoelectricity to ionic conductivity and colossal magnetoresistivity. The structure is built of octahedra BX6 with A atoms occupying large voids in the framework.

The topology of the structure, with octahedra sharing corners, makes the three-dimensional framework very flexible and capable of accommodating A and B atoms of a size varying over a wide range. The mismatch in the size of A and B atoms is readily relieved in such a framework via cooperative distortion without breaking B-X-B connectivity.

A simple way to estimate the compatibility of A, B, and X atoms based on their ionic radii is a so-called tolerance factor . The more the value of t deviates from 1, the stronger distortion of a perovskite structure from the ideal cubic form is typically observed.

It therefore may be expected that the perovskite-type structure of calcium and strontium technetium oxides, CaTcO₃ and SrTcO₃, having t equal to 0.947 and 0.982, respectively, should be distorted. Indeed, in the original paper reporting the structure of SrTcO₃ it is described as a “very slightly distorted perovskite” [1] but no further details were provided.

Since then, the crystal chemistry of technetium containing oxides has been studied only sporadically which is undoubtedly due to the radioactivity of the element and the need to take special precautions when working with samples.

Employing state-of-the-art synchrotron X-ray and neutron powder diffractometers at the Australian Synchrotron, Los Alamos National Laboratory, and ANSTO (ECHIDNA) we have determined accurate crystal structures of CaTcO3 and SrTcO3 over a wide range of temperatures from 4K to 1023K. At room temperature, both materials are orthorhombically distorted (Fig. 1, right).

Figure 1. Ideal cubic undistorted perovskite structure (left) and the orthorhombic structure of CaTcO3 at room temperature (right).

On heating, the distortion gradually decreases and the materials first adopt tetragonal symmetry, and then reach the ideal cubic modification. This behaviour is typical for perovskites and the result would not have attracted much attention, had not we discovered that both materials are magnetically ordered up to ~1000K.

This extraordinarily high temperature of magnetic ordering was completely unexpected, as the isostructural and isoelectronic manganates CaMnO3 and SrMnO3 order magnetically at around 130K and 260K, respectively. Using the neutron powder diffraction data, we determined the magnetic structures of CaTcO3 and SrTcO3 in which each technetium atom is coupled antiferromagnetically to its six nearest metal neighbours, a so-called G-type antiferromagnetic ordering (Fig. 2).

Figure 2. Magnetic structure of CaTcO3 and SrTcO3. The perovskite subcell is outlined with dashed lines.

Although further theoretical studies will be needed to understand fully the nature of the high-temperature magnetic ordering in technetium perovskites, our ab initio calculations within the framework of Density Functional Theory confirm the higher relative stability of the G-type magnetic structure with respect to the non-spin-polarized state and other types of magnetic order.

The calculated electronic density of states reveals that mixing of 4d technetium and oxygen states close to the Fermi level is more extensive than that in the manganates (Fig. 3). This significantly more covalent type of metal-oxygen bonding is believed to increase the magnetic ordering temperature.

Figure 3. Electronic structure of SrTcO3 for non-spin-polarized model (a) and G-type antiferromagnetic model (b) showing extensive mixing of 4d states of Tc and oxygen.

The results of our findings are published in Refs 2-3. At the moment, the crystal chemistry and condensed-matter physics of the technates is still largely unmapped territory and our discovery suggests that further studies of technetium materials will bring more surprises.

References

1. Muller, O., White, W. B. & Roy, R. Crystal chemistry of some technetium-containing oxides. Journal of Inorganic and Nuclear Chemistry 26, 2075-2086 (1964). 
2. Avdeev, M. et al. Antiferromagnetism in a Technetium Oxide. Structure of CaTcO3. J. Am. Chem. Soc. 133, 1654-1657, doi:10.1021/ja109431t (2011).
3.  Rodriguez, E. E. et al. High Temperature Magnetic Ordering in the 4d Perovskite SrTcO3. Physical Review Letters 106, 067201 (2011).