Some Uses of Crystallographic Databases and Bibliographies
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electron count nearly constant, one reSome twenty years ago, it appeared that places Ti(Z = 4) with a mixture of Cu and alkaline earths (as in YBCO). the crystallographic database had grown The second example concerns the chemiso large that it could no longer be described by conventional methods (see, for example, cally simplest material, (Ca,Sr)CuO2, actuStructure Reports).* Today we have available ally a high Tc superconductor with only 8 both computerized databases (CRYSTDAT, four atoms/cell. This was discovered 9 by 2 JCPDS diffraction data, etc.) and excellent one of us after a computerized search of CRYSTDAT yielded BaCuO2, a stable insubibliographies3 which enable us to search lator. This naturally suggested further much larger databases rapidly and far more study of the (Ca,Sr,Ba)CuO2 family to obsystematically, resulting in qualitative tain metastable metallic, and quite likely, changes in methods and results. Following high Tc superconductive alloys. Subseare five examples illustrating this point. quent research10 has yielded Tc — 80 — 100 K in (Sr,Ba)CuO2 alloys prepared by High-Temperature high-pressure synthesis and Tc ~ 16 K in Superconductivity (Sr,Nd)CuO2 thin films.11 The original discovery 4 that Tc [La,Ba,Cu,O] > 30 K, and specifically As a third example related to HTSC, we that the responsible compound was mention that the growth of thin films reLa2-,BaICuO4 with x ~ 0.15, was made quires suitable substrates. We can obtain a by traditional methods beginning with complete list of these from the CRYSTDAT the observation that this combination of database as provided by CISTI.2 We reelements gave a metallic conductivity,3 quire an oxide system, preferably isotropic dp/dT > 0. Much subsequent work was (cubic) with lattice constants matched, for greatly influenced, implicitly if not ex- instance, to the basal lattice constants of plicitly, by inferences from the database. YBCO, a ~ b ~ 3.85(6) A. Restricting the Specifically, after the superconductive search to the intersection of inorganic properties of (La,Ca,Sr) 2 CuO 4 were phases (103,611), then ternary oxides known and Tc a 90 K was discovered6 in (14,299), then cubic phases (2,340), then a ~ YBa2Cu3O7, the chemical pattern of conb ~ 3.86(6) A we finally obtain 108 candistituent elements and the importance of dates. Further reduction follows if we retetragonal structures were obvious. It was quire one of the metals to be a transition evident that it was necessary to include or rare earth element, leaving 37 on the CuO 2 planes in the compound, and list, more (but not all) of which are the fathat these units were rigid and always miliar perovskites. had basalo square lattice constants near 3.82(10) A. The oxide database can be Quasicrystals searched for all such tetragonal oxides, Early examples of quasicrystals were 7 and such a search was done. There are metastable, but eventually it was discovabout 80 such compounds, including ered12 that CuLi3Al6 was stable. The secBi4Ti3Oi2, the parent compound for the ond stable quasicrystal was found13 to
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