High-resolution heat-capacity and thermal-expansion measurements on single crystals of the kagome-staircase compounds (Co 1−x Ni x ) 3 V 2 O 8 are presented. The parent compounds Co 3 V 2 O 8 and Ni 3 V 2 O 8 undergo a sequence of first-and second-order magnetic phase transitions. The low-temperature (T 40 K) magnetic entropy evolves monotonically with the doping content x from the full value expected for Ni 2+ S = 1 magnetic moments in Ni 3 V 2 O 8 to only half the value expected for Co 2+ S = 3/2 moments in Co 3 V 2 O 8 . The thermal-expansion coefficients α i (i = a, b, and c) show a strong anisotropy for all (Co 1−x Ni x ) 3 V 2 O 8 compounds. The lowtemperature expansivities indicate that Co doping (Ni doping) yields changes similar to uniaxial pressures along a or b (c). Linear Grüneisen parameters i are extracted for the three main axes i and are found to exhibit a complex temperature and doping dependence. For each axis, i and α i exhibit a sign change at low temperature at the critical concentration x c 0.25 where the incommensurate magnetic propagation vector changes. Our study motivates further investigations to understand how the multiple and complex parameters such as magnetic frustration, magnetic anisotropy, and mixture of S = 1 and 3/2 ions affect the rich magnetoelastic properties of (Co 1−x Ni x ) 3 V 2 O 8 .
(Co 0.5 Ni 0.5 ) 3 V 2 O 8 represents a mixed compound of the well investigated transition metal (M) orthooxovanadates Ni 3 V 2 O 8 (NVO) and Co 3 V 2 O 8 (CVO) labelled as Kagomé staircase structures, which are characterized by edgesharing MO 6 octahedra isolated by nonmagnetic VO 4 tetrahedra. The crystallographic structure (orthorhombic space group Cmca) [1, 2] is interesting with respect to the magnetic properties as the magnetic ions form buckled planes of corner-sharing isosceles triangles representing an anisotropic variation of the ideal Kagomé net. Within these buckled planes, the Kagomé staircases, cross-tie ions on crystallographic (4a) sites link the linear chains of spine ions on (8e) sites. Due to the reduced symmetry of the Kagomé staircase geometry with respect to the ideal plane net the degree of frustration is lowered leading to interesting long range ordered magnetic structures. Magnetization and neutron diffraction experiments on a (Co 0.52 Ni 0.48 ) 3 V 2 O 8 powder sample [3] revealed only one magnetic phase transition into an antiferromagnetic ground state in contrast to the richness of magnetic phase transitions of its parent compounds [4,5]. The magnetic structure is modulated by a composition dependent propagation vector k=(δ, 0, 0) with δ being 0.491(4) for (Co 0.52 Ni 0.48 ) 3 V 2 O 8 where a similarity to the NVO type magnetic structure was assumed [3]. Neutron single crystal diffraction experiments followed by group theory analysis produced a more detailed picture. The magnetic structure of (Co 0.5 Ni 0.5 ) 3 V 2 O 8 exhibits features, which differ from the predominantly collinear alignment of its parent compounds NVO and CVO, which exhibit a variety of magnetic structures with magnetic moments mainly oriented along the a axis [4][5][6][7]. The averaged magnetic moments of the statistically distributed Ni 2+ and Co 2
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