Which method is used to synthesize HAp?

It is suggested to increase Ca/P ratio slightly above 1.67.

You can use two-step sintering or microwave sintering instead of conventional sintering to avoid remaining long time in high temperature and consequently to avoid TCP phase.

I think si almost impossible to avoid TCP formation at all. Due to energy distribution in side the sample you will have always some amout of TCP as wells as amorphous phase

I agree with the previous opinion. Indeed, it is very difficult (almost impossible) to prepare HAP containing the zero amount of TCP.

Controle its ph above 11 using 28 % ammonia during the reaction.

I am using precipitation method...any other possibility to reduce the TCP content???

The appearing of TCP phase is a consequence of HAp crystal lattice thermal instability, due to stoichiometry ratio of Ca/P lower than 1,67. Have you checked that ratio by chemical methods?

Hydrothermal method is a good one to retain initial precursor ratio in the final material.  You can try at 200 oC or see how we did that in the reference given below.

Article Processing Route to Fully Dense Nanostructured HAp Biocerami...

You should use a very stoichiometric proportion of Ca/P reactants (very close to 1.67).

It is possible to obtain pure hydroxyapatite without any TCP, even if sintered at 1000ºC.

see link bellow for pure HAp: 

http://www.fluidinova.com/nano-hydroxyapatite-powders-for-bone-graft-substitute-biomaterial