every logical volume you are creating is appearing in /dev as a block- and a characterdevice

brw-rw---- 1 root system 10, 12 Dec 09 2005 lvinformix0
crw-rw---- 1 informix informix 10, 12 Mar 16 06:30 rlvinformix0

so assuming you have now created lvinformix1

you should see
brw-rw---- 1 root system 10, 12 May 09 06:40 lvinformix1
crw-rw---- 1 root system 10, 12 May 09 06:40 rlvinformix1

and all you need to do is chown informix:informix /dev/rlvinformix1

afterwards informix should be able to initialize this filesystem ...

Rgds
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