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Raman spectroscopic study of PbCO3 at high pressures and ...

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Fig. 5 Temper<strong>at</strong>ure<br />

dependence <strong>of</strong> the internal<br />

<strong>Raman</strong> mode frequencies <strong>at</strong><br />

7.6(2) GPa: a v 2-out-<strong>of</strong>-plane<br />

b<strong>and</strong> (filled triangle), b v 1asymmetric<br />

stretching vibr<strong>at</strong>ion<br />

(open circle), cv 4-out-<strong>of</strong>-plane<br />

b<strong>and</strong> [1 B 3g (filled right<br />

triangle), 2 A 1g (filled circle),<br />

3 B 2g (filled square), 4 B 1g<br />

(open circle)]. The size <strong>of</strong> the<br />

symbol corresponds to the<br />

errors in <strong>Raman</strong> shift <strong>and</strong> T in<br />

c <strong>and</strong> in T in a, b<br />

The behavior <strong>of</strong> almost all the b<strong>and</strong>s is similar in the case <strong>of</strong><br />

the experiment performed <strong>at</strong> 17.2(2) GPa <strong>and</strong> temper<strong>at</strong>ures<br />

up to 544 K (Fig. 8). Increasing temper<strong>at</strong>ure virtually leaves<br />

the position <strong>of</strong> the b<strong>and</strong>s unaffected, displacements being less<br />

than 1 cm -1 (Fig. 9), <strong>and</strong> the number <strong>of</strong> b<strong>and</strong>s unchanged.<br />

Thus, we conclude th<strong>at</strong> the phase III has already been formed.<br />

Its form<strong>at</strong>ion was accompanied by the occurrence <strong>of</strong> three<br />

new b<strong>and</strong>s (v2 0 , 7, 8) <strong>at</strong> different <strong>pressures</strong> <strong>and</strong> splitting <strong>of</strong> the<br />

v1-symmetric C–O stretching mode <strong>of</strong> the CO3 2- group<br />

(Fig. 8). Direct qualit<strong>at</strong>ive comparison <strong>of</strong> the spectra obtained<br />

<strong>at</strong> room temper<strong>at</strong>ure <strong>and</strong> 17.2(2) GPa (Fig. 8) <strong>and</strong><br />

15.6(2) GPa <strong>and</strong> 323(3) K (Fig. 6), respectively, shows the<br />

same pr<strong>of</strong>ile <strong>and</strong> number <strong>of</strong> b<strong>and</strong>s. This implies th<strong>at</strong> <strong>PbCO3</strong><br />

has the same structure under both conditions. The frequencies<br />

<strong>of</strong> all modes change monotonously with increasing temper<strong>at</strong>ure<br />

(Fig. 9). A change <strong>of</strong> the slope <strong>of</strong> the temper<strong>at</strong>ure<br />

dependence <strong>of</strong> the v 4 internal vibr<strong>at</strong>ions (Fig. 9a) was<br />

observed in the 330(3)–350(3) K temper<strong>at</strong>ure range.<br />

Isothermal (544(3) K) decompression<br />

(a)<br />

Isothermal decompression was carried out in the last set <strong>of</strong><br />

experiments. The pressure was released with 1 GPa steps <strong>at</strong><br />

123<br />

(c)<br />

(b)<br />

Phys Chem Minerals<br />

a constant temper<strong>at</strong>ure <strong>of</strong> 544(3) K. The two additional<br />

b<strong>and</strong>s (marked 7 <strong>and</strong> 8 in Fig. 10), which had appeared<br />

during compression <strong>at</strong> 17.2(2) GPa (Fig. 8b), disappear<br />

below 12.2(2) GPa, as shown in Fig. 10. The disappearance<br />

upon decompression <strong>and</strong> appearance upon compression<br />

(Figs. 1, 2d) <strong>of</strong> the v2 0 out-<strong>of</strong>-plane b<strong>and</strong> takes place <strong>at</strong><br />

about 10 GPa. The v1 0 b<strong>and</strong> disappears just below<br />

12.2(2) GPa upon isothermal decompression <strong>at</strong> 544(3) K<br />

(Fig. 10b). Figure 11 shows the <strong>Raman</strong> b<strong>and</strong>s <strong>of</strong> <strong>PbCO3</strong> <strong>at</strong><br />

544(3) K as a function <strong>of</strong> pressure. The mode shifts <strong>of</strong> the<br />

b<strong>and</strong>s 7, 8 <strong>and</strong> v 4-B 1g are not shown. B<strong>and</strong>s 7 <strong>and</strong> 8 are<br />

virtually pressure-independent. v4-B1g is very weak, in fact<br />

hardly distinguishable from the noise. It is therefore difficult<br />

to determine its precise position. The dv/dP values<br />

are given in Table 3. Mode s<strong>of</strong>tening <strong>of</strong> v2 <strong>and</strong> v2 0<br />

was observed in the whole pressure range (Fig. 11b, d).<br />

The slope dv 2/dP changes <strong>at</strong> <strong>pressures</strong> <strong>of</strong> about 13.0(2)<br />

<strong>and</strong> 7.2(2) GPa. The second change <strong>of</strong> the slope suggests<br />

the return to the orthorhombic starting phase. The same<br />

behavior is observed for v 4 b<strong>and</strong>s marked 1, 2, 3 in<br />

Fig. 11a, namely the same stable value <strong>of</strong> dv/dP in the<br />

15.0–13.0 GPa pressure range <strong>and</strong> change <strong>of</strong> the slope <strong>at</strong><br />

about 13.0(2) <strong>and</strong> 7.2–7.8 GPa. The disappearance <strong>of</strong> the

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