1Certain commercial materials, instruments and equipment are identified in this manuscript in order to specify the experimental procedure as completely as possible. In no case does such identification imply a recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the material, instrument or equipment identified is the best available for the purpose.

2Supplementary materal has been deposited in the IUCr electronic archive (Reference: HM5059 ). Services for accessing this material are given at the back of the journal.