A quantum critical point is approached by applying pressure in a number of magnetic metals. The observed dependence of Tc on pressure necessarily means that the magnetic energy is coupled to the lattice. A first order phase transition occurs if this coupling exceeds a critical value: this is inevitable if diverges as Tc approaches zero. It is argued that this is the cause of the first order transition that is observed in many systems. Using Landau theory we obtain expressions for the boundaries of the region where phase separation occurs that agree well with experiments done on MnSi and other materials. The theory can be used to obtain very approximate values for the temperature and pressure at the tricritical point in terms of quantities measured at ambient pressure and the measured values of along the second order line. The values of the tricritical temperature for various materials obtained from Landau theory are too low but it is shown that the predicted values will rise if the effects of fluctuations are included.
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