Structural stability of W₂B₅under high pressure

Abstract

High-pressure structural stability studies have been carried out on tungsten boride W 2 B 5 up to maximum pressure of 36 GPa using a Mao-Bell diamondanvil cell at beamline BR-12 of the ELETTRA synchrotron facility (λ = 0.68881 Å). The hexagonal phase (S.G:P6 3 /mmc) of W 2 B 5 is stable up to the maximum pressure studied. The bulk modulus is estimated to bẽ 347 GPa using the Birch-Murnaghan equation of state. The variation of lattice parameters and bond lengths B-B and W-B have been studied and the c-axis is seen to be marginally more compressible than the a-axis. IntroductionThere is immense interest in the search for new super-hard materials with improved chemical and thermal stability [1,2]. Among the naturally occurring materials, diamond has the best mechanical properties, but the major drawback with diamond is that it reacts with iron and hence cannot be used for machining steel. The search for materials with good mechanical properties has been in specific covalent substances, covalent and ioniccovalent compounds, and partially covalent compounds of transition metals (TM) with light elements. Compounds formed between TM and light elements like B have wideranging industrial applications owing to their high hardness, high melting point, excellent thermal conductivity, good wear and corrosion resistance properties [3][4][5][6]. The observed superiority of mechanical properties of TM borides, as compared to the nitrides or carbides, is attributed to the 'puckered layer of B atoms' in the boron lattice. Interest in this field has been renewed on account of the recent finding of a hardness greater than 45 GPa in ReB 2 [7]. However, WB 4 has been reported to have a hardness of 43 GPa [8] with a bulk modulus 339 GPa. Also an enhancement of the hardness of WB 4 up to 50 GPa with 1% doping of rhenium has been reported. In fact, tungsten borides offer a more economically viable option in the quest for super-hard materials. Because of wide-ranging interest in tungsten boride systems, various experimental studies and first-principle calculations have been reported [9][10][11][12][13][14][15]. High-pressure structural stability studies have been carried out on WB 4 [8] and the bulk modulus estimated to bẽ 326 GPa. There is considerable debate on the ratio of B to W in WB 4 , and recent