[Pd(NH3)4]Cr2O7
This sample is extremely sensitive to hand-pressure, leading to an extraordinary preferred orientation effect (compare the patterns recorded by pressing the sample on a glass slide or dusted through a sieve, a side-loaded holder giving an intermediate result). Identification is positive by a default search in the PDF-2 database, two cards were corresponding when this job was done a few years ago: 40-1486 and 39-1422. These two cards were absolutely identical before this job, both proposed with a triclinic cell (Powder Diffraction 4, 1989, 217). Since the publication of the structure (Powder Diffraction 10, 1995, 159), the 39-1422 card is marked as "DELETED" remaining however with a "Quality : I" mark, meaning that it was indexed (this early proposed triclinic cell did not passed successfully the test by the Le Bail method, the fit was disgusting). The 40-1486 JCPDS-ICDD card has been rejuvenated by adding the cell parameters included in the recent structure determination. However, the old d(obs) were kept which were indexed according to the new cell ! The corresponding FOM (figure of merit) are of course of very low quality : F30=8(0.028, 140). Really this is amazing because the FOM given in the 1995 paper was F20=51(0.0077, 48) corresponding to new d(obs) which were not inserted in the modified card ! The rules applied by ICDD for the publications of a JCPDS card remain a mystery for me. This case was discussed in the "Rietveld Mailing List'.
We come back to our study. After fruitless research in all databanks made in order to verify if a structure was known for this compound, and after having concluded that the triclinic cell could not be correct, indexing is undertaken. Angular reflection positions are estimated by EVA2. As for Na2C2O4 the zeropoint is evaluated by the method of the harmonics and refining fictitious cell parameters by the ERACEL program (pdcr1.html). The zeropoint is -0.002° theta, this value is reported into TREOR90 (twice this value with a changed sign because the zeropoint is added to the experimental data in 2-theta in this locally modified version). The 20 first reflections observed on the pattern, excluding second or third order harmonics, are selected. TREOR90 is first executed by selecting the high symmetries (orthorhombic or more) without success. Tests in monoclinic symmetry allow to obtain a proposition after having increased the maximum cell volume successively to 200, 400, 600, 800, 1000 and finally 1200 A3. In order to convince yourself that the cell proposition is credible, ITO and DICVOL91 are applied too. ITO does not provide the same cell, even testing the discrepancy parameter with values 0.01, 0.02... and up to 0.06° 2-theta. A cell with twice the volume of the TREOR result is located among the ITO propositions. The ITO proposition with the highest FOM (figure of merit) is triclinic with a volume of 950 A3. DICVOL91 gives the same result as TREOR90, this was expected because DICVOL is the unique really exhaustive program. The dichotomy process explore all possibilities with the disadvantage that no unindexable reflection should be among the data. The TREOR90 and DICVOL91 results are in pdcr2.html. The ultimate verification of the cell quality remains to be done. We have yet seen for Na2C2O4 that this verification may consist in an application of the Le Bail method by FULLPROF. This time too, the P2/m, space group is chosen because no systematic extinction is associated with it. RP decreases to 6% (calculated after background subtraction), giving confidence for further efforts (pdcr3.html). Moreover, examination of the extracted intensities (1127 reflections up to 100° 2-theta), and in the meantime looking at the observed and experimental patterns, allows to suggest the P21/c space group, a centrosymmetrical group without ambiguity. Try by yourself, downloading the full data (pdcr0.zip).
