fixture 16 shown in fig3 the anvil exposed ends 30 have a diameter of 0 5 inch in place of the washer 26 a thick retaining ring 36 with a central opening having upper and lower tapered surfaces 38 40 and a circular vertical central portion 42 of 0 560 inch in diameter is used the upper and lower tapered surfaces 38 40 mate with similarly tapered surfaces on the upper and lower anvils 22 24 prior to assembly electrically insulating pressure sealing tapered rings made of pyrophillite 46 are placed between the surfaces to be mated and an insulator 48 is placed between the upper and lower surface of the assembly and the pistons 12 14 of the press 10 when the fixture is assembled a circular reaction chamber 34 of 0 560 inch in diameter and 0 150 inch thickness is created between the components using prior art techniques in which the reactants are reacted under ambient conditions for extended periods of time tacl 5 has been reacted with li 3 n to produce hexagonal phase tan with only a trace of the cubic phase using the process and apparatus described above and operating at approximately 30 kbar or greater the same reaction produces primarily cubic phase tan with only a trace of the hexagonal phase it has been found that the process and apparatus described above can also be used to synthesize products which are not thermodynamically favored using the same reactants at ambient conditions gan can not be formed by solid state metathesis reactions under ambient.