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Low-temperature behavior of NaGaSi₂O₆

F. Nestola^1,*, N. Rotiroti^2,, M. Bruno³, M. Tribaudino⁴, S. van Smaalen², H. Ohashi⁵ and G.J. Redhammer⁶

¹ Bayerisches Geoinstitut, University of Bayreuth, D-95440, Bayreuth, Germany
² Laboratory of Crystallography, University of Bayreuth, D-95440, Bayreuth, Germany
³ Dipartimento di Scienze Mineralogiche e Petrologiche, Università di Torino, Via Valperga Caluso 35, I-10125, Torino, Italy
⁴ Dipartimento di Scienze della Terra, Università di Parma, Parco Area delle Scienze 157/A, 43100 Parma, Italy
⁵ HASHI Institute for Silicate Science Nishinakanobu, 1-9-25, Shinagawa, Tokyo 142-0054, Japan
⁶ Institute of Crystallography, University of Technology, Rheinisch-Westfälische 11 Technische Hochschule Aachen, Jägerstrasse 17/19, D-52056 Aachen, Germany

Synthetic NaGaSi₂O₆-pyroxene, space group C2/c, was investigated at low temperature between 295 to 110 K by X-ray diffraction. The evolution of the unit-cell parameters as a function of temperature does not indicate any structural phase transition down to 110 K. Calculated values for the Debye temperature, _D, and the Grüneisen parameter, , are 653(39) and 0.84(8) K, respectively, close to those of other pyroxenes. Five complete intensity data collections were performed at 295, 235, 190, 145, and 110 K to investigate the temperature dependence of the crystal structure. The space group remains C2/c down to 110 K, but anomalies were found between 235 and 190 K for the temperature dependencies of the volume and strain of the M1 polyhedron. This anomalous behavior could be related to variations of the Ga-O2(C1,D1) bond length and the O1(A1,B1)-Ga-O2(C1,D1) bond angle. The C2/c-P phase transition observed in a previous work for NaTiSi₂O₆ at about 200 K did not occur for NaGaSi₂O₆ down to the minimum temperature investigated in this work. This is not due to the different ionic radius at M1 site or to a different tetrahedral chain extension between these two compositions but it is likely due to unfilled t_2g orbitals, which do not exist in NaGaSi₂O₆.

Key Words: Clinopyroxene • crystal structure • X-ray diffraction • low temperature

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