Negative thermal expansion and magnetic ordering in double tungstate Li2Ni(WO4)2
Sunil K. Karna1*, C.-W. Wang2, R. Sankar1, M. Avdeev3, I. Panneer Muthuselvam1, F. -C. Chou1
1Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan
2Neutron group, National Synchrotron Radiation Research Center, Hsinchu, Taiwan
3Bragg Institue, Australian Nuclear Science and Technology Organisation (ANSTO), Sydney, Australia
* presenting author:Sunil K. Karna,
The family compounds of double tungstates have been studied in recent years, because of their interesting luminescence properties and possible application in the field of solid-state lasers.
We report here on the negative thermal expansion and magnetic ordering in double perovskite Li2Ni (WO4)2 using neutron and x-ray diffraction, magnetic susceptibility, specific heat and magnetization measurements. Li2Ni(WO4)2 crystallizes into Triclinic P-1 symmetry. The thermal variations of magnetic susceptibility and specific heat show anomalies at 12.5 and 17.5 K. Long-range commensurate antiferromagntic magnetic order of Ni spins develops below TN = 12.5 K when the crystalline unit cell experiences thermal expansion upon cooling. No direct connection between NiO6 octahedrons is seen indicating the magnetic coupling involves a more complex path like Ni-O-Li-O-Ni within the a-c plane or Ni-O-W-O-Ni between neighboring layers. The negative thermal expansion seen in the magnetic ordered state that is associated with the WO6 octahedral distortion. No long-range magnetic order is detected when bulk sample is reduced to nanoscale. The saturated magnetic moment that we obtained for the Ni2+ ions at 2 K is <μz> = 2.45(6) μB. The Ni spins lying in the a-c plane inclines at 24.5o with the a-axis.

Keywords: Double tungstates , Negative Thermal Expansion, Magnetic ordering