Isochoric temperature differential of the x-ray structure factor and structural rearrangements in low-temperature heavy water

X-ray structure factors for heavy water were measured at temperatures of 40.0, 23.5, 15.5, 11.2, 7.1, 0.2, and -11.0^∘{}C. From these data isochoric temperature differentials (ITD) were constructed for pairs of temperatures ΔT=40.0-(-11.0)=51.0,23.5-0.2=23.3,and 15.5-7.1=8.4 ^∘{}C around the maximum density point at 11.2^∘{}C. We show that the temperature dependence of the structural parameters can be obtained much more precisely from ITD than from the direct determination of the structure factor. A major structural rearrangement at the second-neighbor shell is identified from the interpretation of the ITD. We then interpret this rearrangement in terms of a possible increased molecular angular correlation of the tetrahedral structure at low temperatures.