LESS THAN STANDARD SEPARATION INCIDENT WHEN LGT DESCENDED THROUGH ALT OF SMT; FREQ CHANGE MISSED BY LGT. INCORRECT POSITION RELIEF BRIEFING.

Narrative

LGT X WAS GIVEN CLRNC THAT PUT HIM IN CONFLICTION WITH SMT Y THE RECEIVING CTLR RECOGNIZED THIS AND TOLD ME TO STOP LGT X 1000' ABOVE SMT Y. I AGREED THAT LGT X WAS IN CONFLICTION WITH SMT Y AND SHOULD BE STOPPED 1000' ABOVE; HOWEVER I HAD ALREADY TRANSFERRED LGT X TO THE RECEIVING CTLR. AS IT TURNS OUT; LGT X NEVER ACKNOWLEDGED THE FREQ CHANGE AND DSNDED THROUGH SMT Y THE SECTOR WAS BUSY MOST OF THE DAY AND IT WAS AT SHIFT CHANGE WHEN THE ERROR OCCURRED. I BELIEVE IT WAS A CLASSIC CASE OF LETTING ONE'S GUARD DOWN A MOMENT TOO SOON. I HAD TO BE PAGED BACK TO THE SECTOR AS I WAS WALKING OUT OF THE BLDG. CLEARER COMS BTWN SECTORS COULD HAVE PREVENTED THE ERROR. ENSURING PLT ACKNOWLEDGEMENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: CALLBACK TO DETERMINE LOCATION OF THE INCIDENT AND THE TYPE OF ACFT INVOLVED. SEP WAS 3 NM AND 200'. RPTR WAS WORKING A COMBINED POS OPERATION. EXPERIENCE LEVEL WAS 7 YRS AND TFC VOL WAS LIGHT. INCIDENT HAPPENED IN THE ATHENS SECTOR AIRSPACE. RPTR HAD BEEN RELIEVED AND GAVE HIS RELIEF A BRIEFING; BUT ALSO HAD TOLD HIM THAT THE LGT HAD MADE A FREQ CHANGE. RPTR WAS ON HIS WAY OUT OF THE BLDG AND WAS PAGED BACK TO THE CTL ROOM AS THE INCIDENT WAS CHARGED TO HIM.