Tetrahedral Structure Lab Report

Paragraph 1

In iron phosphate, the iron atoms and phosphorus atoms are coordinated to the oxygen atoms in a tetrahedral arrangement. As there is much space present in this structure, it can thus expand or contract as temperature changes. This paper studies the contraction and expansion of the structure of the iron phosphate molecule as temperature increases from

294 K to 1073 K. The tetrahedral structure, which is the structure of α-quartz, is adopted at lower temperatures. As temperature increases, the structure changes in a phase changing process. The first transitional change occurs when temperature reaches 980 K.

As temperature increases from 294 K to 980 K, increase in the iron-oxygen-phosphorus bridging angles is evident. Besides this, tilt angles in the tetrahedral structure decreases and

Although occurring in a nonlinear fashion, thermal expansion in the α phase is strong as the cell parameters (Fig. 1) and volume (Fig. 2) of the iron phosphate molecule increases with increasing temperature. The volume data here follows a thermal expansion coefficient α (K-1

) = 2.924 x 10-5 + 2.920 x 10-

10 (T-300)2

.

As we enter β phase however, there is no thermal expansion as there is an absence of angular variations, which was the contributing factor to the strong thermal expansions in α phase. These angular variations are in turn due to the changes in iron-oxygen-phosphorus bridging angles and tilt angles as mentioned above.

As temperature increases and the structure transitions from α to β, the tilt angle δ changes as it follows the Landau-type model (δ2 = !

"

δ0

2 [1 + (1 – "

# (T–Tc/T0 –Tc))

%

&]). In this model, δ0 refers to the decrease in tilt angle at 980 K while Tc refers to the temperature at which the second-transition occurs.

The behavior of iron phosphate is distinct from that of other α quartz as the δ tilt angle