Hydrogen niobate

In the preparation of lithium niobate by CVD, argon-containing oxygen is used as a carrier gas. In the preparation of mercury telluride, the carrier gas was hydrogen. Suggest reasons for these choices of carrier gas. 

Barium titanate is one example of a ferroelectric material. Other oxides with the perovskite structure are also ferroelectric (e.g., lead titanate and lithium niobate). One important set of such compounds, used in many transducer applications, is the mixed oxides PZT (PbZri-Ji/Ds). These, like barium titanate, have small ions in Oe cages which are easily displaced. Other ferroelectric solids include hydrogen-bonded solids, such as KH2PO4 and Rochelle salt (NaKC4H406.4H20), salts with anions which possess dipole moments, such as NaNOz, and copolymers of poly vinylidene fluoride. It has even been proposed that ferroelectric mechanisms are involved in some biological processes such as brain memory and voltagedependent ion channels concerned with impulse conduction in nerve and muscle cells. 

Niobium pentafluoride forms colourless, highly refractive prisms density 8 2932 at 18° C. It melts at 75 5° C.f and boils at 217° to 220° C. under a pressure of 760 mm. of mercury. It is extremely hygroscopic and deliquesces rapidly in air. It is reduced by hydrogen at 286° C. in contact with platinum to an unstable lower blue fluoride. Excess of concentrated alkali hydroxide or alkali carbonate solutions attack it with formation of the alkali niobate. It dissolves in toluene, paraffin, carbon bisulphide, and other organic solvents. 

The monoxide has been variously described, according to the method of preparation, as a black powder, or glistening crystals, or beautiful black cubes. Its density varies from 6 3 to 6-7, and it is a good conductor of electricity. On being heated in air it oxidises readily to the pentoxide with considerable evolution of heat. It dissolves in hydrofluoric acid and hydrochloric acid with evolution of hydrogen, and a pentavalent niobium salt is formed in solution. It is also dissolved by boiling potassium hydroxide to form potassium niobate. This behaviour of niobium monoxide towards acids and alkali indicates that divalent salts of niobium are too unstable to exist. A divalent chloride of tantalum has, however, recently been isolated (see p. 192). 

After being strongly ignited, niobium pentoxide becomes insoluble in all acids other than hydrofluoric acid, but is dissolved by molten potassium hydrogen sulphate, ammonium hydrogen sulphate, and borax. It is also insoluble in solutions of alkalis, but is converted into the alkali niobates by fusion with alkali hydroxides and carbonates. 

Oxalo-niobates or niobo-oxalates correspond to the vanado-oxalates, and contain both oxalic acid and niobic add radicals in the complex anion. The only known series possesses the general formula 3R aO. Nb 205.6C203.a H20, where R stands for an alkali metal. The sodium, potassium and rubidium salts are prepared by fusing one molecular proportion of niobium pentoxide with three molecular proportions of the alkali carbonate in a platinum crucible. The aqueous extract of the melt jjs poured into hot oxalic add solution concentration and cooling, or addition of alcohol or acetone, then brings about precipitation of the complex salt. Comparison of the dectrical conductivity measurements of solutions of the alkali oxalo-niobates with those of the alkali hydrogen oxalates determined under the same conditions indicates that the oxalo-niobates are hydrolysed in aqueous solution, and that their anions contain a complex oxalo-niobic acid radical.6... 

Potassium Pemiobate, K3Nb08s is prepared by the action of caustic potash and 3 per cent, hydrogen peroxide on 4 3 potassium niobate. It yields tetragonal crystals. 

Rubidium Pemiobate, Rb3Nb08, is obtained as a white, crystalline powder by the action of rubidium carbonate and excess of hydrogen peroxide on 4 3 rubidium niobate. 

Sulphides.—Definite sulphides of niobium have not been prepared. Solutions of alkali niobates do not yield precipitates with hydrogen... 

Oxysulphides.—The action of carbon bisulphide vapour or hydrogen sulphide on niobium pentoxide, sodium niobate or niobium oxytrichloride gives rise to a black powder which assumes a metallic appearance on being rubbed, and which conducts electricity well. This is probably an oxysulphide of niobium, Nba.OJ,Sz, but its exact composition varies with the experimental conditions, and its individuality is a matter of doubt.5 Oxysulphides of niobium of doubtful composition, mixed with potassium sulphide, are also produced by the action of carbon bisulphide vapour on potassium oxalo-niobate. ... 

The application of semiconductor-liquid junctions is of special interest for the direct production of a chemical fuel. Especially the production of hydrogen by photoelectrolysis of H2O has been studied by many research groups (compare with [114,194]. It has been demonstrated by many authors that H2-formation is rather easy at semiconductor electrodes. The crucial point is the simultaneous oxidation of H2O. So far, photoelectrolysis was only achieved with SrTiOs, a semiconductor of a large bandgap (Eg = 3.1 eV) [206]. Very recently, photocleavage of HjO was also found with some niobates under open... 

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