Abstract
One of the many unusual properties of water is a minimum in the isothermal compressibility at 46.5oC. At higher or lower temperatures the structure of liquid water changes providing less resistance to pressure. Here we present high energy x-ray diffraction data over a wide range of momentum transfers that shows the 1st oxygen-oxygen shell expands linearly and has a constant coordination of 4.3(2) (measured up to 3.3Å) over the temperature range -19oC to 93oC. The second peak in the oxygen-oxygen pair distribution function, displays a continuous structural transition, concomitant with the compressibility minimum at 46.5oC. The least compressible form of water at ambient pressure is associated with an average nearest neighbor O-O distance of 2.810(5)Å and an average O-O-O angle of ~108o.
