title: Investigation of Antisite Defect Formation and Chemical Expansion in LiNiPO4by in Situ Neutron Diffraction.
abstract: In situ neutron diffraction was used to characterize the effect of temperature on the crystal structure of LiNiPO4. LiNiPO4adopts an ordered olivine structure at room temperature, but, with increasing temperature, this work shows that a significant amount of Li and Ni cation exchange occurs, for example, ∼15% at 900 °C. The antisite disorder is detected by residual nuclear densities on the M1 and M2 octahedral sites in the olivine structure using difference Fourier maps and by changes in cation site occupancies, lattice parameters, and mean ⟨M-O⟩ bond distances. The antisite disorder is also responsible for chemical expansion of the crystal lattice in addition to thermal expansion. Antisite defect formation at high temperature and its reversibility on cooling can be understood as an entropically driven feature of the crystal structure of LiNiPO4. The lithium ion diffusion pathway, that follows a curved trajectory along the b axis in the olivine structure, is, therefore, susceptible to be blocked if synthesis conditions are not carefully controlled and should also be influenced by the chemically expanded lattice of the disordered structure if this is preserved to ambient temperature by rapid cooling.
authors.affiliation: Catalonia Institute for Energy Research , Jardins de les Dones de Negre 1, 08930 Sant Adrià de Besòs, Spain.
authors.forename: Jordi
authors.identifier: http://orcid.org/0000-0001-5981-6168
authors.initials: J
authors.lastname: Jacas Biendicho
authors.affiliation: Department of Product Development, SYNergy ScienTech Corp , 7F, No. 9, Park Avenue ll, Hsinchu Science Park, Hsinchu 30075, Taiwan.|Department of Materials Science and Engineering, University of Sheffield , Sheffield S1 3JD, United Kingdom.
authors.forename: Kuang-Che
authors.initials: KC
authors.lastname: Hsiao
authors.affiliation: ISIS, STFC Rutherford Appleton Laboratory , Chilton, United Kingdom.
authors.forename: Stephen
authors.initials: S
authors.lastname: Hull
authors.affiliation: Department of Materials Science and Engineering, University of Sheffield , Sheffield S1 3JD, United Kingdom.
authors.forename: Anthony R
authors.initials: AR
authors.lastname: West