Apparently continuous isosymmetric transition in ammonium hexafluorophosphate NH4PF6

NH4PF6 undergoes two transitions at ambient pressure on cooling: Fm (3) over barm-C2/c and an isosymmetric C2/c-C2/c transition. Isosymmetric transitions must be discontinuous. However, while this transition displays a latent heat, there is little sign of a discontinuity in neutron powder diffraction or nuclear magnetic resonance, consistent with being close to a Landau critical point. The practical implications of slightly discontinuous isosymmetric transitions are discussed.