KH2PO4 in a Field: A transition without critical microscopic fluctuations

Light scattering on potassium dihydrogen phosphate demonstrates that the ordering electric field reduces the isothermal microscopic response. At the continuous transition occurring at the critical field no microscopic critical fluctuation exists (no diverging correlation range) and only macroscopic thermodynamic quantities can diverge classically. This general effect for symmetry-nonbreaking elastic transitions is caused in potassium dihydrogen phosphate by higher than bilinear strain-polarization couplings in the free energy. © 1979 The American Physical Society.