Nb and Ta are geochemically related elements which typically give rise to isomorphous series of oxides as accessory minerals in granitic pegmatites (e.g. columbites, pyrochlores, fergusonites, etc). However, in spite of the natural occurrence of orthorhombic alumotantite (AlTaO4), its Nb counterpart is only known as a synthetic product. AlNbO4 crystallizes with monoclinic symmetry (s.g. C2/m) and was early described with a strict ordering of Nb and Al, respectively over M1 and M2, the two non-equivalent octahedral sites in the structure [1]. This model was revised by successive refinements showing that a partial disorder occurs in AlNbO4 with a ~20% inversion degree [2,3]. CrNbO4 is a disordered MM'O4 compound, crystallizing with the rutile structure (s.g. P42mnm) while FeNbO4 crystallizes with wolframite (s.g. P2/c), orthorhombic α-PbO2-type, rutile, or AlNbO4-type structure as the temperature increases. This brief review shows that, even at room conditions, the chemical nature of the metal cations M3+ strongly affects the crystal structure of MM'O4-type oxides. In order to better understand the substitution mechanisms, the cationic ordering, the structural stability, and the phase transition processes for (M3+)(Nb5+)O4 compounds, six samples along the (Al1-xCrx)NbO4 join (x: 0.0–0.5) were synthesized by the solid-state reaction process and investigated by means of a X-ray powder diffraction (XRPD) and electron absorption spectroscopy (EAS) combined approach. Monophasic up to x(Cr)=0.125 apfu (AlNbO4-type structure), the samples along the join become biphasic (the former plus the CrNbO4-type structure) for x(Cr)>0.125 apfu, indicating the impossibility to have a complete solid solution between (Al,Cr)NbO4 oxides. The lattice parameters increasing with the Al–Cr substitution defines a structural expansion limit (x(Cr)>0.24 apfu) that is ascribed to the maximum chromium content that the structure can accept. The samples up to x(Cr)=0.25 also exhibit the typical optical spectra of Cr3+ in 6-fold coordination, involving d-d electronic transitions. Ascertained that the proposed octahedral ordering models were corrected [2,3], through different refinement strategies, the global instability index, GII [4], was calculated over the un-doped structure by varying the Al/Nb ratio in the two metal sites. Unpredictably, even the minimal values of GII obtained are very close to the limit of 0.20 v.u. that implies strained structures. With a crystal field strength that decreases moving from x(Cr)=0.06 to 0.50 apfu, and both the Racah parameters and the nephelauxetic trends nearly constants, no significant change in the degree of covalency and polarization of the Cr–O bond are found. The 10Dq decreasing suggests an elongation of the local Cr–O distance for Cr3+ increasing at site M2 and to a minor extent at site M1, fitting well the structural data on the long-range mean bond distances. The degree of structural relaxation around Cr3+ was also assessed by means of the relaxation coefficient ε, until the structure will maintain the same symmetry. The AlNbO4 lattice has a limited propensity to relax (ε≈0) and the AlNbO4 structure follows the Vegard’s law. In contrast with the well known Al–Cr joins in garnets, spinels, perovskites and corundum, the AlNbO4 structure is strongly constrained by other factors, like cation ordering and electrostatic charge balance. Indeed, the M2 site hosting Al3+ is underbonded and Cr3+ can be accommodated at this site without significant strain for that the lattice does not need to relax around the bigger ion. [1] Pedersen B. (1962) Acta Chem Scand 16,421; [2] Efremov V., Trunov V., Evdokimov A. (1981) Kristallografiya 26,305; [3] Greis O., Ziel R., Garcia D., Claussen N., Breidenstein B., Haase A. (1996) Mater Sci Forum 228,825; [4] Salinas-Sanchez A., Garcia-Muñoz J., Rodriguez-Carvajal J., Saez-Puche R., Martinez J. (1992) J Solid State Chem 100,201.

Structural properties of Al1-xCrxNbO4 join: a XRPD and EAS combined approach

ARDIT, Matteo;CRUCIANI, Giuseppe
2011

Abstract

Nb and Ta are geochemically related elements which typically give rise to isomorphous series of oxides as accessory minerals in granitic pegmatites (e.g. columbites, pyrochlores, fergusonites, etc). However, in spite of the natural occurrence of orthorhombic alumotantite (AlTaO4), its Nb counterpart is only known as a synthetic product. AlNbO4 crystallizes with monoclinic symmetry (s.g. C2/m) and was early described with a strict ordering of Nb and Al, respectively over M1 and M2, the two non-equivalent octahedral sites in the structure [1]. This model was revised by successive refinements showing that a partial disorder occurs in AlNbO4 with a ~20% inversion degree [2,3]. CrNbO4 is a disordered MM'O4 compound, crystallizing with the rutile structure (s.g. P42mnm) while FeNbO4 crystallizes with wolframite (s.g. P2/c), orthorhombic α-PbO2-type, rutile, or AlNbO4-type structure as the temperature increases. This brief review shows that, even at room conditions, the chemical nature of the metal cations M3+ strongly affects the crystal structure of MM'O4-type oxides. In order to better understand the substitution mechanisms, the cationic ordering, the structural stability, and the phase transition processes for (M3+)(Nb5+)O4 compounds, six samples along the (Al1-xCrx)NbO4 join (x: 0.0–0.5) were synthesized by the solid-state reaction process and investigated by means of a X-ray powder diffraction (XRPD) and electron absorption spectroscopy (EAS) combined approach. Monophasic up to x(Cr)=0.125 apfu (AlNbO4-type structure), the samples along the join become biphasic (the former plus the CrNbO4-type structure) for x(Cr)>0.125 apfu, indicating the impossibility to have a complete solid solution between (Al,Cr)NbO4 oxides. The lattice parameters increasing with the Al–Cr substitution defines a structural expansion limit (x(Cr)>0.24 apfu) that is ascribed to the maximum chromium content that the structure can accept. The samples up to x(Cr)=0.25 also exhibit the typical optical spectra of Cr3+ in 6-fold coordination, involving d-d electronic transitions. Ascertained that the proposed octahedral ordering models were corrected [2,3], through different refinement strategies, the global instability index, GII [4], was calculated over the un-doped structure by varying the Al/Nb ratio in the two metal sites. Unpredictably, even the minimal values of GII obtained are very close to the limit of 0.20 v.u. that implies strained structures. With a crystal field strength that decreases moving from x(Cr)=0.06 to 0.50 apfu, and both the Racah parameters and the nephelauxetic trends nearly constants, no significant change in the degree of covalency and polarization of the Cr–O bond are found. The 10Dq decreasing suggests an elongation of the local Cr–O distance for Cr3+ increasing at site M2 and to a minor extent at site M1, fitting well the structural data on the long-range mean bond distances. The degree of structural relaxation around Cr3+ was also assessed by means of the relaxation coefficient ε, until the structure will maintain the same symmetry. The AlNbO4 lattice has a limited propensity to relax (ε≈0) and the AlNbO4 structure follows the Vegard’s law. In contrast with the well known Al–Cr joins in garnets, spinels, perovskites and corundum, the AlNbO4 structure is strongly constrained by other factors, like cation ordering and electrostatic charge balance. Indeed, the M2 site hosting Al3+ is underbonded and Cr3+ can be accommodated at this site without significant strain for that the lattice does not need to relax around the bigger ion. [1] Pedersen B. (1962) Acta Chem Scand 16,421; [2] Efremov V., Trunov V., Evdokimov A. (1981) Kristallografiya 26,305; [3] Greis O., Ziel R., Garcia D., Claussen N., Breidenstein B., Haase A. (1996) Mater Sci Forum 228,825; [4] Salinas-Sanchez A., Garcia-Muñoz J., Rodriguez-Carvajal J., Saez-Puche R., Martinez J. (1992) J Solid State Chem 100,201.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11392/1860710
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