"been claimed to exist" is correct and usable in written English.
You can use it when you are describing something that is rumored or suggested to exist but is not necessarily a confirmed fact. For example: Some evidence has been found that suggests the existence of a lost city, though its actual existence has only been claimed to exist.
Despite the existence of Xe VIII), no Rn VIII) compounds have been claimed to exist; RnF8 should be highly unstable chemically (XeF8 is thermodynamically unstable).
The higher fluorides RnF4 and RnF6 have been claimed to exist, and are calculated to be stable, but it is doubtful whether they have actually been synthesized.
[ 11- 16], but they are also marketed to prevent or reduce some of the reproductive aspects of ruminant Q fever that have been claimed to exist such as metritis, retained fetal membranes, infertility, sterility, mastitis and increased herd prevalence of abortion and stillbirth.
He will make a judgment based on what is in the final legislation, not what is claimed to exist in a press release, or media report".
Thus, no ultimate truth is affirmed (or rather, nothing is claimed to exist ultimately).
This forms an 'organisational improvement trajectory' (Steiber and Alänge 2015; 12), which is claimed to exist on all organisational levels.
They therefore place a limit on the closeness of the relationship that can be claimed to exist between attention and consciousness.
It's almost impossible to tell when a classic investment bubble has formed — some major bubbles went almost completely unpredicted, while other bubbles were claimed to exist without any measurable consequences.
Despite the official policy of free ANC services in public HFs in Tanzania, some HWs reported that user fees for ANC service were claimed to exist and that such fees excluded or delayed the poorest women from seeking ANC services.
The last captive died in the Cincinnati Zoological Garden in 1914, but the last generally accepted observation in the wild was a flock seen in Florida in 1920, although it has been claimed that they existed in South Carolina until 1938.
A reward offering of US$12,000 has also been claimed, though no documentation exists for that figure either.
