Improve ment of t he X -ray diffraction pattern file of t he America n Society fo r T esti ng M aterials is t he bas is for work being don e at t he N ational Bureau of Standards wi t h t he cooperat ion of t he Joint Co mmi ttee on Che mical Analysis by X -ray Diffraction Methods of t he Ameri can Society for T est ing Materials , t he America n Cr ystallographic Associa t ion, an d t he Bri tish Inst it ute of Phy sics. The equipment used in obtainin g t he d ata presented her e comprises a Geiger-count er spectrometer wit h a 180 0 ar c calibrated with t he calculat ed patterns of substances whose lattice const ants are known with high pr ecision . Separate chart s are prep ared to ob tain dat a for interplanar spa cing and intensity m easurements so t hat special attention can be paid t o mounting t he sample for t he best result s in each case. Tables are in cluded t o com pare t he patterns with t hose recorded in the literature.T ables a nd discussion follow for X-ray diffraction dat a ba sed on fou r compounds not listed in t he Ameri can Society for T esting M aterials file and on four represented there. T he pa tterns of zinc borate, Zn B20 4 ; selenium oxide, Se0 2; m agnesium silicate (forsterite ), M g2Si0 4 ; and magnesium t un gstate, Mg W04 , a re recommended as additions to t he file. The patterns of st r ontium nitrate, Sr (N 0 3h; bari um ni trate, Ba (N 0 3h ; lead fluochloride (matlockite) , PbFCI; and mcr cu ric chl oride, H gCh, are reco mmend ed to replace t he present patterns in t he fil e.

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Forty-six standard X-ray diffraction powder patterns are presented. Fourteen are to replace twelve patterns already given in the X-ray Powder Data File, and thirty-four are for substances not previously included. The X-ray Powder Data File is a compilation of diffraction patterns from many sources and is used for the identification of unknown crystalline materials by matching spacing and intensity measurements. The patterns were made with a Geiger counter X-ray diffractometer, using samples of high purity. The d-values were assigned Miller indices determined by comparison with calculated interplanar spacings and from space group considerations. The densities and lattice constants were calculated, and the refractive indices were measured whenever possible.

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A silicon powder Standard Reference Material, SRM-640, has been prepared for use as a standard in powder diffractometry. Powder diffraction measurements were performed with a tungsten internal standard and a high-angle goniometer. The measured a/2 is 3.525176. With 2(Cu Kel peak) taken as 1.5405981 ~, a= 5.430880 (35) ,X, uncorrected for refraction. Comparison of a with values obtained with a single crystal from one of the boules reveals a difference of 3 parts in 105. This difference suggests a subtle systematic error in powder diffractometry or a change in lattice spacing near crystal boundaries. Use of the SRM should permit individual measurements of lattice parameters to be made reproducible to near 1 part in 105 and an absolute accuracy of at least 3 parts in 105.