It should be noted that, when the CSP was performed under the same conditions by adding 20 wt% of 1M HNO3 solution instead of yttrium nitrate solution, Y4(OH)9NO3 was formed as second phase (data not shown). The same reaction is expected to occur in yttrium nitrate solution because the Y2O3 particles are dissolved and react with HNO3 solution. The chemical reaction for ion formation in the yttrium nitrate solution and phase formation during CSP at 200℃ can be expressed as follows: